UIToolkit is Unity’s new system for creating and editing UIs for games and the UnityEditor. Like Flexbox4Unity (this asset) it is based on CSS3-Flexbox. Start here for UIToolkit-specific help: https://flexbox4unity.com/2021/02/25/2021-uitoolkit-all-tutorials/
However, as of 2021, Unity’s version is not yet ready for production use and has a lot of bugs and missing features. Because I’ve spent many years researching and building Flexbox implementations I often find bugs in Unity’s UIToolkit and figure out workarounds – I then write them up here for people who are using UIToolkit in their own apps.
(of course … where possible I’d recommend you use UnityUI + Flexbox4Unity instead – it’s a much better combination!)
Right now the only situation in which I recommend UIToolkit is Editor-Scripting: if you want to customize the UnityEditor you cannot use UnityUI (sadly) – Unity decided not to allow it. So you’re stuck with UIToolkit.