The main cause of a flat or house heating up are the windows. A friend of mine who lived in Japan for a long time told me how they deal with the extrem heat. The most important thing is to block the south and west facing windows from the sun

from the outside! Normally in Germany most old houses have outside blinds for that reason. Unfortunately newer buildings don’t, in order to save money.
In Asia they hang bamboo blinds outside in front of any glass windows. These are cheap and very effective and she told me, saying they keep at least 80 percent of the heat out of the room.
I have ordered 4 blinds for my south facing huge patio windows and also some sticking hooks especially designed for outside.
All together this was affordable and they will arrive before the next heatwave

I will post the blinds in case anyone is interested.
Another idea is too wet the concrete (south facing) outside of the windows and doors. This must be done in the early mornings or late evenings.
My friend sent me a video explaining these precautions, I can share it if anyone would like to watch it.
Even if you have ac with these easy tricks you can save a lot of money