I don't remember many details so you have to wait for more competent replies but afaik there are 2 ways of getting extensions working on UGC: download the extension file using some workarounds and install it manually, or install a special proprietary module that enables Chrome extension store so you can use it like on Chrome.
Agree with a lot of the comments here, whether it's privacy or not wanting to use GAFAMmmy tools, there are better options than ungoogled chromium. For privacy: https://privacytests.org/ (maintainer works at Brave, but by and large this site is useful)
If I were you and needed to use a chromium based browser, I would go for brave with maximum privacy settings turned on and all the Crypto shit and brave "features" turned off. Ungoogled chromium is a good project but its not as refined as Brave is. See here: https://privacytests.org/
If I were you I'd go for Librewolf or Mullvad browser if you dont need Chromium. The reason people say this is because Google has a sort of monopoly on the browser space right now as Chromium takes up Almost 80% of the marketshare, meaning that Google will ultimately get to decide what features live, and what features die. If Chromium market share was down at 40% or so, it wouldnt matter if you used Brave or not, but thats the reality we live in.
If you turn off the telemetry like I said in my comment that no longer becomes an issue. In terms of blocking your digital fingerprint Brave is going to be better than every popular Chromium based browser. See the link I posted in the original comment.