Followup: OP, based on your post update, you do indeed mean bind mounts. Okay.

As I said above, what I would probably do myself for most cases is to just use a symlink, not a bind mount. That's easy, doesn't require futzing around with the system, and in most cases probably does what you want, whereas a bind mount may not (e.g. if you have a filesystem indexer or backup system, they're not going to back the stuff up or index it twice).

That'd be something like:

$ ln -s /mount/external/realfolder /home/user/folder

That does mean that /home/user/folder will still be visible when you don't have the media inserted, though it'll just be a dangling symlink.

If you definitely do want to do it with bind mounts...hmm. It looks like there are some additional hurdles.

https://unix.stackexchange.com/questions/799166/linux-auto-mounting-usb-with-udev

According to a comment there, it looks like systemd-udev shouldn't run a mount command directly, as it runs commands in a private namespace; it should use systemd-mount --no-block, which will ask systemd to set this up. You could probably write a bash script that runs a series of commands, including the systemd-mount command. It may be a bit involved for a linux4noobs solution, and I can't give an off-the-cuff answer from personal experience, but I'd guess that it may very well be technically possible, and that's probably what I'd be looking into if I were determined to specifically to use bind mounts.