How to select the explorer when ui.allow_platform_file_picker is true.
You can lookup relations with file and process on /usr/share/applications/default.conf .
If you wish to change a relation or to add a relation, You can override above file with to change ~/.local/share/applications/defaults.list .
Default explore is defined by the entry called "inode/directory".
To change nautilus to pcmanfm for example.
[Default Applications] inode/directory=nautilus.desktop
[Default Applications] inode/directory=pcmanfm.desktop