Questions from noon

So I downloaded this game and it’s cool and I want to get into it but I have no clue what to do. The ui is confusing and I don’t know what my goal is. I see a few different game modes that I don’t really understand. Also, it seems like every mech is already unlocked which I don’t really like. What do I unlock as I play? New weapons or something? I want there to be something I can works towards. Is there a main game mode I should focus on?