Yes, those are the normal IDs for extensions, and yes, they are meant to be that way.
if you sideloaded an extension, the ID will be generated automatically based on the path, so if you always use E:\extension\ Chromium browsers will always have the same ID in your computer, so it is done automatically by their system to avoid duplicates.
For example, you can easily check Chrome extension store.
About Brave extensions like mnojiijdmmbfmejpflffifhffcmidifd well, you can easily check that here brave://inspect/#extensions.
That Brave extension is what controls Web Discovery Project, which is the way to easily help Brave indexer to be improved without you doing nothing but only let Brave pretty much gather pages you visit and searches you make.
Also, that extension has the element Picker for the Adblocker. In the past, it also had the Shields Panel, but when ShieldsV2 was released, then it was made a native solution, like the Android shields panel, and removed as an extension.
If you for example start the browser with --disable-brave-extension you will not use Web Discovery Project, and you won’t see the Element picker, and well, in the past you wouldn’t even see the Brave shields icon/panel.
I don’t understand why the drama and saying stuff like, “suspicious-virus-looking names”, when it is normal, extensions have to have an unique ID to avoid issues, especially when most extensions get updated or might change name and all that. I mean, would that be different it was just numbers? no, it would be the same, Google just opted to use letters instead of numbers.
Also, about PreferenceMACs well, nobody is sure what that’s for, you will see they say you can transfer profiles just by using those, but it is a lie, because if you remove the registry entries and open your browser, the browser doesn’t get affected and actually re-make the same registry entries but with different values, so maybe it has to do with the IDs and maybe generated by system with the Encryption key and all that, but it is not something that is meant to be relevant for anything in the browser, just programs using registry for their information.
If you see Secured Preferences file, you will see many values like that, and in the extensions for example, it will not match what you see in the registry. Removing the ones in registry will not cause any issue but touching the ones in the User Data will.
So I think you should just let it be and ignore it and stop overthinking about it, it’s just how things work, especially in Browsers that are supposed to give security and all that.