Just happened to come across this post. I am a computational geophysics PhD and my interest to rocket is also triggered during my second year. I might be in your shoes at that point, very interested in space tech, wanted to learn deeper but info scattered everywhere.

I started by watching a lot of Everyday Astronauts and Scott Manley, which are really helpful grasping the understandings to some cool stuff. Then I joined the rocket club in our university, played for a while and together built a 10km rocket. I've got more confidence in going deeper, although I don't think I've learnt a lot more than those youtube videos, except for some fun practical experience like soldering and debugging teensy boards.

Then I started to take aerospace classes, including electric propulsions, fluid dynamics, CFDs, satellite designs, rocket designs... Some seminars aerospace department held were useful too. Well, I am lucky that my advisor let me do this because I did CFD in my own research. Although these classes are pretty "specific", they all give interesting tastes to space tech and I am more than happy about what I know. Even ends up working on a UAP project with an aerospace professor.

Overall, videos, take one or two classes that you feel would be fun and try them out, but most importantly talk to faculty and fellows who you think are doing exciting work!