What’s the difference between the main series and the 1st film?

You mean Der film?

Turns out the agreement made between the government and Oorai to keep their school open if they win the tournament was only verbal and therefor not legally binding. Anzu therefor makes the government representative sign a legal agreement to keep oorai open if they beat a university team in an annihilation match.

It’s similar to significantly different because of it being a single battle and all the high school teams, all come together

Pretty sure the first film is them fighting the college team or whatever with the prodigy girl and then the series is the entire tournament for tankery against the schools we know and new ones

The film they do everything they can to avoid what happens in Der Film.

Das Finale is set after Der Film.