After some "dry exercises" it was finally time: The Raspberry got its new task as a "Smart Home Central"...

Here are some steps I did to get everything up and running:

1.  At first I build everything up and checked for electrical functionality
2.  After this I used a a clean new install of "Raspbian Buster with desktop"
3. Manually created wpa_supplicant.conf because the Raspberry Pi 3A+ does't have a ethernet connection and I wanted to configure the Pi remotely (even if you are using a touch display, there is no virtual keyboard pre-installed) wpa_supplicant (German)
4. Installed remote desktop protocol:
   1. sudo apt install xrdp
   2. sudo reboot
5. Installed virtuall keyboard
   1. sudo apt-get update
   2. sudo apt-get install at-spi2-core florence
   3. sudo reboot

After this it was time to read out some temperature data from the BMP280:

1. Installed Adafruit blinka
2. Installed Adafruit BMP280 Lib