For new designers, I always recommend starting with About Face: The Essentials of Interaction Design. It’s a doorstopper of a book, but contains more or less everything you need to get started on the UX/interaction side.
To polish UI technical skills, one of the best ways is to just get in repetitions. Try copying stuff off Dribbble and figuring out how they got certain effects done. Notice spacing, alignment, color, and how all these combine to create hierarchy and draw attention to the desired parts of the design.
Good luck!!
UI and UX are two different things
i'd suggest to check what others are doing so you can develop a taste
there are resources like Dribble and many prominent UI & UX teams have their own blogs (like https://design.google)
then, learn some CSS, don't waste time on Figma when you can just prototype in dev tools
there's also a book which shows good/bad by example: https://www.refactoringui.com/book