The general way to master a large pile of difficult abstractions is to study for years, ideally in a context focused on that study. This
can be done independently of any expert feedback (e.g. by reading books, watching lecture videos, and doing a big pile of ungraded self-assigned homework problems), but it’s not easy or typically very efficient. For the majority of people who want to deeply learn mathematical topics but who are starting from a low level, the most efficient and effective advice for them is to go attend a university.
An online discussion forum isn’t typically going to cut it, IMO. People won’t be at the same level, won’t be working on the same topics, won’t have the same goals and interests, the same level of commitment, coordinated schedules, etc. This has been tried many times, but I have not heard many successful reports.
Online discussion forums are great for handling independent limited-scope questions–answers (including for current university students), and okay for coordinating research efforts among people who are already experts, but in general are not very effective for getting a group of people to diligently work through years of prerequisite material in an organized fashion.
You shouldn’t spend your life in a university, but if you have a goal of learning advanced mathematics, you might benefit from spending a few years there. YMMV.
As an alternative, if you can find an expert tutor/coach to meet with one-on-one on a regular basis, that is even better than a formal course. But that is expensive and hard to scale.