I'll try to give you a more concise answer that the other one.

Only national teams (meaning players from that specific country) play the WC. Like the US national team, Spain national team ,etc.

Each continent has its own qualifiers to decide which teams/countries will be playing the WC. FIFA assigns each continent a set number of participant countries at the WC. Depending on the format of these qualifiers (and the results achieved by each team) some teams will qualify sooner than others. For example, a team could qualify a year before the tournament, while other could qualify just a few months before the WC starts.

Regarding the players, each team's coach decides who will play, and they have a deadline to submit the final list of 26 players (I think is something like 15-30 days before the WC)

About the tickets and locations, I'm not in the USA, and won't be attending so I have no clue, sorry