Kerbal Space Program is pretty great for getting to understand a rather simplified version of orbital mechanics and aerodynamics (no n-body physics unless you install Principia and no realistic aerodynamics unless you install FAR). Just don’t buy KSP 2.

For mods, you should use CKAN to get them. The comments in this Reddit thread have some good recommendations. You can also play with the RP-1 modpack + Principia if you want an ultra realistic experience:
https://github.com/KSP-RO/RP-1/wiki/RO-&-RP-1-Express-Installation-for-1.12.5

Edit: You should only use Principia with the Real Solar System mod (which is included in RP-1) because n-body physics breaks the game with KSP’s vanilla star system.