I havn't look tutorial on UI for a while,
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has some good one for previous version (3.5). Some functions and properties were renamed between 3.* and 4.* but for most part it should give you a good (and kinda deep) view on Control nodes, which are all UI related classes availables.
I learnt mostly from this playlist
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it's for an inventory but he used many Control, explaining why and how, etc. Really good stuff.
For basics, Godot documentation is quite good, you might need to have it always open if you try to follow a 3.* tutorial.
I'm currently working on an arpg and I've downloaded ClydeDialogue (from the AssetLib) for the dialogue parts, it seem a good option. The guy has its own dialogue scripting language, the addon is an interpreter and automate almost all tasks. You can write and test your interactions within his addon, then, in game, you just need to call a start function with a file path and it work well (hmm, it work well for my little tests. For now I have only integrated it and run a couple of sentences each time)
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to see how dialogue are written in this language.
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for info about Godot port.
Also, Godot Café and Godot Engine discord are massive help!