Get ready, noob questions incoming!
So I have this dongle and the inverter is the exact version of this tutorial.
My question starts with: what cable should I buy? And then, where should I plug it in the inverter? Also, what if I want to connect other devices (let’s say, a AC for instance). Should I simply put the other cables in this same USB converter or should I use a new one for every device I want to connect to OpenHAB? I’m using RPi4 w/ OH3.3
Noob questions ended. 
You haven’t explained of what kind of connection you are trying to make. I will start from beginning as you point a rs485 dongle.
With short distance and a bit of luck anything will work (ie. CAT-5), with longer distance you need to switch to a “transmission” cable with matching nominal impedance of 120 Ohm (its also a value of terminating resistor in ideal scenarios). You can look for “modbus” cables and their parameters, look for example there https://www.fscables.com/products/modbus.html.
For inverter side - look for instructions. There should be place for RS485 wires, but it might be under some removable cover for safety reasons.
With regard to AC (air conditioning), its a question if your unit has a modbus interface (rs485 does not imply modbus). If it does then you can try to go over single wire. On openHAB end it will be then a single serial connection going to several slaves (unit ids). If you don’t know what you doing and AC happen to have modbus interface try each individually and only then think of making a chain. RS485 is a bus, so you can plug multiple participants to it as long as you do not branch cable. It must have a linear topology, not a star.
You’re right, my bad.
I am looking to connect both the inverter and the AC units all into that single RS485 dongle, which will then be inserted into a USB HUB connected to my Raspberry Pi 4 8Gb, working with OpenHAB v3.3 installed in an SSD.
What I want to achieve, is from the inverter only read information, but from the AC I want to read and write, being able to control each unit independently.
For the inverter, since it is exactly the same version as the one in this tutorial, I only need to know what cable I should go for (RPi is somewhat far away from the inverter, some 50m distance) and where to connect it. Also, what if I mess-up connections, can that “frie” something or with RS485 there isn’t that risk and things will only not work?
How can I know this for sure? I’ve seen these cables are not cheap so I don’t want to spend time and money buying something that is not suitable.
Actually, the unit does not have one, but I do have a quotation to put one there. But now you said something very interesting. I can connect the RS485 directly and avoid the usage of a modbus at all? If that’s the case, it would save a good money! :o
Would this work also for the floor heating as well? And where should I look for it and what should I expect to find?
Thank you so much for the quick answer and sorry for many questions. With these things I just don’t want to commit any mistakes at all so I go very, very carefully and making sure I am not wasting time, money nor breaking anything.
I think your posts deserve separate topic as its not strictly related to this tutorial but planning of installation. Let’s hope moderators will helps us to move it around. 
What you try to do is rather common, as you can’t /shouldn’t parametrize inverter until you know exactly what you doing. Normally these tasks are reserved for electricians and other staff.
For 50m I think its safest to go with a proper cable to avoid troubles with proper termination of lane. The rs485 can go over 1000m but with low speed, for 50m you should be fine with 9600 baud.
If you go to electric warehouse you can ask about “data” cable and tell them for what you plan to use it (modbus, rs485) or give them the cable characteristics from earlier page. Guys should know then what to offer you. I’ve seen few of these cables and they usually have a short info on themselves with ie. impedance “120 ohm”. Typically they are green or purple, but do not buy them for color but for parameters. If you buy from online store - look for parameters. Sites such as TME (online electronics distributor) and others I think can sell you these cables too.
Risk of firing anything is not high, but there is always some. Overall rs485 is low voltage bus (afair its −7V to +12V), so its not dangerous for human life, yet if you cross this wire with a higher voltage then you will can get into a troubles. 
As far I could read from dongle spec you sent it seems to have some ESD protection, but I am not sure if its isolated. Inverter side should also have some protection, but that’s something you need to verify with inverter card/technical instruction. ESD is about electrostatic discharge, when there is spark coming from one thing to . Some dongles (more expensive ones) are isolated so they don’t have a direct line between a plug and computer plug (they usually use optocouplers). Any stroke/crossing on the wires will damage receiver side but not computer where dongle is plugged.
RS485 is a physical layer (like ethernet), modbus is protocol (like http). This means that RS485 does not imply a concrete protocol. Sometimes people use it in place of modbus, but it is not the same thing. For example you can use RS485 to run BACnet/MSTP on top of it. BACnet mstp and Modbus are incompatible. To be fair, it might be likely that your AC unit will be easier supporting above than modbus as BACnet is/was popular in HVAC segment.
If you look for equipment then try to keep protocol list short as it will give you least maintenance. Modbus is pain to setup but once completed works reliable over long time.
Thank you very much for your kind help!
I will definitely open a new post for this “project”.