I can't start my MacBook Pro 2017 using controlR, controloptionR, or shiftcontroloptionR.
For all combinations I only get the folder with a question mark shown.
What can it mean and how can I fix it?
This is pretty easy - the folder with a question mark means no bootable volume is connected to the computer despite what disk used to be the startup disk was in the past.
You'll want to connect to a WiFi or wired network and try booting to internet recovery or bring a known good bootable OS to this machine so you can boot to disk utility and see if the internal drive needs first aid or a wipe and erase.
To manually start up from macOS Recovery over the Internet, hold down Option-Command-R or Shift-Option-Command-R at startup.
Barring that, you'll need to seek hardware service if you don't have internet boot or a way to bring a bootable OS to this Mac.
The only one thing to try is resetting the NVRAM one time. Hold Command - Option - P - R at boot and make sure you hear the startup chime twice before letting go.