First, did you review and use the Getting Started Tutorial? Getting Started is the user’s guide and the rest of the docs are the reference guide.
If yes, pointing out specific places where something is missing or unclear in that guide would be the most useful way to help improve the situation.
Note, my replies below are not intended to be RTFM. They are pointing out where I think what you are struggling with is covered in the docs to help identify where they need improvement.
First “Thing” and “Item” are terms of art with a specific meaning in OH. Do you really mean “point it to the thing”? If so indeed your understanding is incorrect. Widgets only display Items, never Things.
This should be covered in the concepts section of the docs: Concepts | openHAB (this is the first page but I’m referencing the whole section).
How to add a widget to a Layout Page is spread throughout getting started but it’s not covered as a specific topic. That could be one way to improve this.
At a high level, if you want to control how stuff resizes and flows on different sized screens you’d add some rows and columns. Then you click the black +icon on a row to add a widget, select from the list. Then click the widget’s black config icon to set the widget’s properties. Here is where you’d point the widgets at the Items.
Not really. There have been some additions and new features, but over all MainUI has largely remained the same in structure and overall use since OH 3.0.
But there’s not just “the” UI. Not counting the phone apps there are three UIs. You do need to be careful when reading off the forum that what you are reading is relevant to the UI you are using: sitemaps/BasicUI, HABPanel, or MainUI. These are completely separate and independent UIs.
Are you, for example, finding sitemap stuff and trying to apply that to MainUI? The different UIs are brought up in Getting Started at Pages - Introduction | openHAB. That’s not deprecated stuff. That’s a wholly different UI.
If it’s in the forum 
Who’s going to volunteer to read every post made and add deprecation notes? If someone wants to volunteer to do that we won’t stop them off course. But I’m not going to do it. There are hundreds of thousands of posts.
If it’s in the docs, it’s not deprecated. Anything that truly becomes deprecated will almost always generate warnings in the logs. A new “better” way does not necessarily deprecate the old way. “Better” is in the eye of the beholder.
Specifics here would help.
In Getting Started? Semantic Model | openHAB
This whole section describes this step-by-step. Did you miss it or misunderstand it? If the latter, specifics can help improve it.
We have many but more are always welcome. Take this as a call for more people to post more tutorials.
But there are many in the Tutorials and Solutions section of the forum already.
I know I’ve written many such over the years, but we can always use more. Justin just posted an excellent tutorial on best practices for developing custom widgets.
This is what Getting Started is for. It shows how to use OH. Everything else is the low level reference.
There’s a whole page on it in Getting Started. Semantic Model | openHAB.
If you want a really low level explanation see A Deep Dive into the Semantic Model.
I totally agree, and there are always at least one it two PRs open and being actively worked to address this at any given time. But we are also up front that this stuff is complicated, and it takes a lot of work.
They have the advantage of having a company backing them with paid developers. They can say “Hey! Joe Smith! Go implement X.”
We have to wait for a volunteer.
It depends. If you create a semantic model, everything that is in the semantic model will be visible and controllable from the Locations, Equipment, and Properties tabs of the Overview page. No additional work is necessary. Those just build themselves.
If you want a custom Page, that’s going to be a little more effort.
But if you don’t want to tackle that right away, Sitemaps are much easier to create. You could then use MainUI for admin only and use BasicUI for display. It’s less flexible and configurable but because of that it’s much easier to create.
Without specifics there isn’t a whole lot to recommended for help.
But the scale issue I can address. Click on each widget’s config icon and select “column options”. There you can set what percentage of the row that widget consumes for different screen widths. If there isn’t room, the next widget will start a new row.
This is covered here: Responsive Layout Pages | openHAB.
Did you read the IP Camera add-on docs? Or are you trying to display feeds straight from your cameras without the add-on?
The docs are always going to be the first place to look. If you still have problems or questions, a new post is preferred. As far as I’m concerned, no one should read everything. Make it clear you made at least some effort to find an answer, and if the answer is posted somewhere someone will post a link to that. If not someone will help.
Without specifics . Some bindings are harder to get working than others due to the technology involved. I’d put the IP Camera add-on near the top.
A lot of people use something like Frigate to manage the cameras. I think there’s a beta Frigate add-on on the marketplace.
But I’m successfully using RTSP so all I can say is it is possible to make it work. But there are lots of variables that can cause problems.
pay
It all depends on how the developer of the widget wrote it. All the custom widgets are community developed and offered. They are not created by nor maintained by the openHAB project. Consequently, their quality and complexity varies widely.
I don’t know this widget specifically so can’t comment. A lot of the time a user will create a widget so you can just provide items that follow a certain naming scheme, and you just need to provide the first part of the name. But other times you have to identify each Item individually.
Note, widgets know nothing about Things. They can’t see them nor can they interact with them. It’s all Item based. And all the relevant Items may not come from the same Thing. That’s by design and a very powerful feature of the OH architecture.
I haven’t looked at HABPanel in years so can’t address that. I also don’t know anything about the semantic home widgets except to say that, like the astro widget you found, are community developed and maintained.
This part I can help with. You install it from the marketplace. Add an instance of it to a Layout Page. Then configure the properties. Based on the little I do know about it, you’ll point it at an Equipment Item from the semantic model, and it will do the rest.
Once last thing I want to point out is on almost every page from MainUI you can open the developer sidebar. The fourth tab of that will have context sensitive step-by-step tutorials and links to the relevant parts of the docs.
If we have any way to make improvements though we need to know ,did you read Getting Started and misunderstood the relevant sections, or did you not read it? I ask because a lot of the stuff you bring up as not being in the docs are in fact there. I’m never going to claim they are perfect and can’t be improved, but they won’t help you if you don’t read them.
The tutorial is designed to be an end-to-end flow through configuring OH. Its purpose is to provide the middle level of the docs. If you read it and didn’t understand, please let’s get into specifics so we can generate actionable suggestions. If you didn’t read it, please do so and come back with any questions left unanswered.
I think that page is generated dynamically based on the number of reads of the post or some other criteria. I scanned through the first dozen it so and almost all of them are still relevant to OH 5. Even the ones that start with OH 3. I didn’t go into the details so there may be some minor details that might have changed. But over all, these things haven’t really changed. But it could be worth an issue to the openhab-docs repo to curate the list a little more to bring the more recent tutorials to the top of the list.
As for more up to date tutorials, all I can do is reiterate the call for volunteers to write them. Also for those who have written them to update them as necessary.
It’s all a community effort.