After you put your disc in your optical drive and MakeMKV does an initial read, click the backup icon instead of the normal rip-a-movie workflow. The backup icon is along the top, it is a yellow folder with a green arrow pointing into it.
It will copy the contents of a disc to your storage. It'll be the standard folder structure in the case of a blu-ray or UHD and an .iso file in the case of a DVD.
Later, MakeMKV can read the contents of the backup to produce .mkv files if you so wish.