Quite all right.
That was my error because I did not understand that you were working in fedora since the output from rubby only showed PopOS entries.
Now that you have said you are trying to get fedora to boot both with the allowed timeout then we can work with that.
First, you would probably need to have the grub menu displayed at all times.
That can easily be done with sudo grub2-editenv unset menu_auto_hide so it does not automatically hide the menu when booting.
Next set the timeout as desired. Edit the /etc/default/grub file and set it to 15 for a 15 second delay, then run sudo grub2-mkdonfig -o /boot/grub2/grub.cfg to make that effective.
Then reboot to test both changes.
Now let us know the details of what else you would like to change and we can work on that one at a time.