How to add topics on iOS?

I remember when I downloaded the app it asked me what topics I’m interested in, (grammar, sports etc) and added those to my Library page.

there were a few I didn’t click that I now want to add to my Library, but I don’t see any option for that.

secondly, I also liked a few of my favourite courses, hoping to stick them all in the same place for reference - but I don’t see them anywhere. Is there anywhere I can see my likes all together?

You will need to select topics on the website version under the Settings > General.
You can’t see lessons you liked but all the lessons/courses you opened and studied previously are listed under the Continue Studying shelf on the home page. If you expand the shelf you can use available filters and sort by options to find them easier.