I’m a Linux dinosaur user since mid 00’s and I confirm that despite huge efforts to make it as seamless as possible, it still sucks today. The problem is that you even have different file pickers (that’s what xdg-desktop-portal tries to mitigate but some applications will do it the traditional way by including toolkit library and filepicker from it, or they will even implement their own), there’s a great freedom to how drives can be mounted and multiple systems to manage drive mounts. It’s managed by gvfs or kio or something else, the behavior is a little differently every time. There are attempts to handle all automatic mounts in /run/media and while most distros conform to that, some won’t.
What I would recommend is to
create your own mountpoints for your internal drives that you don’t expect to change too frequently. It’s done in /etc/fstab. If you’re on KDE, the Partition Manager app can help with setting mount points.
your primary desktop file manager (like Dolphin, Nautilus or Caja) probably has option to copy absolute file paths. Sometimes copying them is easier
If you see GNOME’s file picker, the path is hidden unless you know magical combination of CTRL+L that shows and allows to edit the path