The twrp.app is not usefull anymore, in the old time it was for updating an already installed TWRP
fastboot commands don’t works on Samsung devices,
you need to use Heimdall or Odin instead
No,
Heimdall (or Odin) is needed to flash the custom recovery.img (or .tar).
Then, you will need to boot in a custom recovery mode to install /e/, using adb sideload (or the TWRP install feature)