No luck with google and reading manuals, so here the question:
Platformio takes Sketch.ino and library files - all placed on the Rasberry, to compile the firmware.hex file, and then upload to an Arduino (platformio run --target upload). That works fine.
(and the the ‘clean’ command)
However I prefer to :
a. compile the large Arduino sketch (and a dozen large libraries) in my PC and create the firmware.hex file there,
b. then only send the hexfile to the remote Rasberry via Internet.
c. use SSH commands to let platformio upload the firmware.hex file, without local compiling.