Reddit reviews University Physics

PBS Space time is a good place to get your feet wet.

You need a solid foundation in mathematics and physics; at the very least the first, but preferrably 2-3 years worth of university level studies. Something like this is covered over the first year when studying physics: it's an introduction of the essential topics, which are then developed over the 2nd and 3rd year.

Introductions / first year studies in astrophysics are generally another 500-1000 pages, but I don't have a reference in english.

General Relativity

Quantum Mechanics
The Stanford University channel also offers lecture playlists on cosmology and general relativity.

As if the above isn't intimidating enough, I'll finish with an opinion that this could easily be ~5 years worth of active studies, depending of course on the starting level of the student. If math is already your second language, I can imagine one could start a PhD project after three years.

I'm not referring to sources on the math side, but generally there are two approaches (depending on the university): you study math from the math department, or you study math from the physics department. My path was the former, but the latter would've worked better for me. People differ in this regard, some of my fellow students thrived with the more rigorous maths treatment we were getting. Me, I was failing with "easy" proofs & stuff in maths tests while having little to no problems with utilizing mathematics as a tool on the physics courses.

Disclaimer: finished physics studies 20years ago, been digging deeper into ~all of the above as a hobby ever since.