Help us get you back up and running as quickly as possible by clearing up a few things (see here for examples of the kind of information that really helps us).
It seems you are on OH3 now if you are talking about UI configuration issues, but what version of OH3? How did you upgrade?
There’s lots of information about the new MainUI in these forums and in the docs.
If you’ve read through all of those doc pages and still have questions then, by all means, put those questions up here on the forum and we’ll try to get you squared away. But, we’re going to assume that you’ve at least read through those docs first and start our explanations at that level.
Perhaps that has not been your way in the past, but many people, myself included, that previously relied exclusively on file-based configurations have transitioned over to using the UI for the majority of the work because the new UI makes it fairly clean an easy. You might find it worth at least giving it a fair try to see if all the new improvements fit your use cases. There are plenty of pros and cons for each option, it just depends on what you need. I’m just suggesting that you not close that door by default; there’s probably a lot more on the other side of it than you are expecting at first.
Anything you create through the UI is saved in a json format in one of the files in the /userdata/jsondb directory. Sitemaps specifically are found in the uicomponents_system_sitemap.json file. However, because this is a json format it will not look like the sitemap files you are accustomed to.
Which code? Which online tool?