How to erase a FileVault encrypted hard drive and then reinstall macOS

I should know this next part but I don't. Regarding step 7 of the document you linked. How would I erase the Hard Drive if I'm booted onto that hard drive's system??? Do I boot into the "recovery partition" which I've never done??? Or do I clone the drive, then boot from the clone so I can then erase the original drive???

That's one...

Then once we've gotten that part, say I have an older MacBook Pro (2009) that can't run Catalina. I would guess it might have stopped at say High Sierra or something like that??? How do I access an older version of macOS (are they all downloadable going back quite a ways???) and install from what??? Again, do I have to be booted on some other drive or hold down some key that puts me in some startup mode that will offer me the choice to install an OS on a drive???

I've done this before but it's been a least 10 years so I need the "how do I do step 7" version 101... So, any help???

Got it... Sorry, I should have looked a bit deeper myself and would not have then used your time... Think I just got a bit lazy there... Thank you very much for the help...

Oh, and I take it by what you said about the disk encryption that there is no need to turn off File Vault. Instead just leave it on and do the deletion work, eh??? Does FileVault then just stay on the disk even if you empty it waiting for the next data to come along and get encrypted on the fly??? thanks...

Got it... What are your thoughts on simply erasing disks and then recycling for perhaps some next guy to have the old computer??? I have a 2009 and a 2013 MacBook Pro I want to recycle. The 2013 is SDD and runs Catalina (APFS). The 2009 is a HDD (two in fact one I once put in in place of the CD (remember CD's???) and is HFS+ I think.

Apple did away with the ability to overwrite drives what 1, 7 or 35 times or whatever it was?? when they went to SSD's because they said the way files get stored on SSD's, that method of eliminating data doesn't really work or something like that??? My guess is I will be able to overwrite the 2009 HDD's if I choose to go to that trouble. But what about the SSD's where you can't do that??? Would just erase and be happy??? I just have family business type stuff on the drives that I erased long ago but not yet the entire drive... And such erased is recoverable I think if someone was so inclined.

What is your level of comfort regarding how you choose to remove data from these newer SSD drives for the purpose of recycling a Mac??? Or would you open the computer and destroy the SSD??? I've had that thought... And then just recycle the carcass. Your thoughts on data security when recycling??? thanks...

thanks for all the help... Much appreciated...