This guide page goes into what an alternative UI is, and some of the popular ones that are available. It is a little out of date, lemmy.ca also dropped tesseract we are no longer hosting Tesseract for lemmy.ca

https://fedecan.ca/en/guide/threadiverse/alternative-uis

The controversy around tesseract is better explained elsewhere, but it boils down to a questionable blocklist that the dev was injecting into the tool, without it being made apparent to the users or admins