Testing OS 11 big sur questions

I want to test OS 11 before I commit to it.  So, I would put it on a SSD USB3 stick and run it from there.  Presently I run OS 10.15.7.

Now, reading here https://support.apple.com/en-us/HT201475

I see under point 6 that it tells me …..and related updates to your Mac firmware.

All good and well.  However, if I do run it on an external SSD I suspect the firmware will be updated on the machine I run the install from.  The question is, will I be able to reboot in OS 10.15.7 after that?  Does anybody know? 

Anything else to look out for?