To clarify: aye, obviously you can use /etc/fstab to mount things on macOS. By default it's empty; but that doesn't mean you cannot create it, it just means that, by default, there are no (external) ...
Boot to macOS Catalina and enter the following commands.
diskutil apfs resizecontainer disk0s2 0
sudo diskutil mount disk0s1
rm -r BOOT
rm -r ubuntu
diskutil unmount disk0s1