In high school you can choose to join cross country or track as an extra thing, is that true for when you are in college? If no, then how does it work?

no, sports in college are serious. track and cross-country are like any other college sport, most athletes have scholarships, but you can try to walk on.

Well l walked on to my college team just because l wanted to, but the person above me is right - in college it is very serious. Yes, u probably could walk on, but u also need to prove yourself. There is no slacking in college. If u do not work, u wo not do well (or stay on the team very long)
Also, if u run cross country, u also have to run track. There is no picking & choosing.
Those who walk on sometimes end up with scholarships as well, so u could if ur times r good.
If u r planning on running in college, l suggest u look up the number of the coach & give him a call, or have ur high school coach call him (or her). Good luck!