Grid systems…whether pre-set or self-rolled are pretty easy once you understand them.
There are no set columns that restrict you just that each class sets a specific width. They all float (usually) so if you need an element that’s 1/4 the width of the space you would usually give it a class of .col4 (where 4 = 4/16).
This means that you can have a div next to it that is no more than 12/16 wide. (.col12?)
This works with either set pixel widths for the base ‘column’ widths or %.