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