[NiklasMort]

I'd start with this video to get an overview of all the topics and areas that mathematics entails (some might be unknown to you) https://www.youtube.com/watch?v=OmJ-4B-mS-Y . Then you go ahead and research a bit what sounds interesting to you and then you might google that topic and add "foundations" to that google search. It's just that most school/university math is heavily focused on analysis and algebra but there is so much more!