How does it work with the F1 with the cars?

I'm currently watching the documentation about the 2019 Formula 1 season on Netflix. It is noticeable that many racing teams have problems with their cars, which apparently only becomes apparent in qualifying or, worse, in the race itself. Are teams not allowed to make changes to their cars or test drives between races? Or is it just too expensive? Are the cars tested at all? If you look at this documentation you really get the impression that this is not the case.

There are 6 test days in barcelona before the season (was in the last 2 weeks) and some test days after the races in the season. Between the races you can of course always convert the car as long as the changes are in accordance with the rules.

Test days are prohibited during the season. Before the season there are a total of 6 in which all teams can participate.

Everything else is only tests for the tire manufacturer… So not to develop the car…

Each team can develop their car as long as they want. However, it is not that easy since all teams already have a very GOOD basic concept and only make optimizations. It's also not that easy to catch up during the season, since everyone else also brings updates