1. Did Codesys Target is writing something on SD card after uploading plc program during normal work? I am considering about lifetime of sd card in raspberry pi.
only if you write the bootproject from CODESYS on download of the application
2. If i would like to use trends on VISU, where those data are stored? (SD card or RAM)
by default it is in: /var/opt/codesys/PlcLogic/Visu/
you need to redirect the location where the files are stored.
3. Is there any option to store trendlog data on USB stick or on eg. NAS server?
Keep in mind that if you update the runtime these file is set back to it's original values.