I will try my amateur explanation again, it was not contradicted last time!
Android is carefully built and secured to do all the consumer facing jobs which Google can say are secure enough for a bank or any secure organisation to be able to assume the person typing on the keyboard is their customer without a doubt.
Some free thinking people, like many readers here, do not want to be part of that. LineageOS and /e/ in slightly different ways allow us to unlock the system and put in a “cleaner” one with surprising ease.
One reason the answer is not sounding quite straight is that different manufacturers implement the Android security features slightly differently. When you ask the OP question of one particular device you can get a more well defined answer!
Having unlocked it can we just lock it back up again? Well, not without making good all the missing pieces of the lock, perhaps including a Google presence. Put it another way, re-locking the device would attempt to “turn the key” but are all the internal components there, or is there a booby trap?
If the device would lock, the install guide writer would have included it.
Check out other alternative ROM builders. This forum has a few topics worth study. I deliberately include the link to the very well informed author on DivestOS first.
DivestOS vs. /e/ OS - security and privacy easy - #83 by anon88181694
Is it possible to re-lock bootloader of Moto G7 Plus? - #5 by piero
https://edevelopers-blog.medium.com/microg-what-you-need-to-know-b213bf03565e
https://community.e.foundation/search?expanded=true&q=lock%20bootloader