booting into D/L or recovery mode

Restarting your mobile device into its Download mode (primarily only on Samsungs), Recovery mode,or Safe Mode does involve using different buttons from a cold start but which buttons vary depending on the make and model - i.e. for most Samsung devices it required simultaneously holding down the Volume Up, the Home (or Bixby for newer), and the Power buttons to get into Recovery mode and Volume Down, Home (Bixby), and Power to start up on Download mode. But now a lot of devices require you to boot up into Fastboot mode to access other modes - i.e. with some Moto devices at a cold start hold down the Volume Down and Power buttons. Basically there is no universally established button sequence to reboot into the different modes, you need to go online and find the appropriate pattern for the particular mobile device you're using.
As for your question can they be lost in a bad flash, yes. The button sequence to access the modes is set in non-writable firmware in the mobile device itself but the software is part of the ROM. So if you've flashed a custom Recovery like TWRP and rooted your device, then later decide to re-flash with a stock ROM, that will reinstall a non-rooted OS along with a stock Recovery. (The button sequence won't change, that's hardware/firmware, but the software gets wiped/replaced.) If you intentionally or inadvertently try to flash a ROM for a different model and/or flash the wrong Recovery, it will most likely soft-brick your device and re-flashing with the correct ROM should restore the device to working condition again. But that's all very conditional on a lot of things -- i.e. if you flash the wrong Recovery, flash the wrong ROM, wipe the partition table -- than yeah, it could result in a hard-brick. But emphasize 'conditional' because you might still be able to restore the device after hard-bricking it. Not likely and very dependent on which model (every model has a different hardware configuration inside).

 

Ahhh, OK. What I commented on previously doesn't apply as it pertains more towards smartphones/tablets in general. This might be more relevant:
https://www.hardreset.info/devices/samsung/samsung-gear-live/