For me it all depends on the columns in a row in the first place! For every col I make I set "col-12 col-lg-x", so they'll be full-width on mobile and will take certain amount of space (1-12) on large screens. You also need to experiment with padding (p-x) and margins (m-x) as they can also be set differently on PC and mobile. The minimum height and h-100 is also useful in different situations when you need, for example, the image in a card to be larger on PC but smaller on mobile. In overall, it all strongly depends on the layout and how you want everything to look like on mobile. 

Usually I make sure the layout is mobile friendly right when I code the blank template with no content inside yet. You can use th.circlejourney.net to code your stuff live and switch between normal mode and "vertical" time to time so you can see how everything might look on mobile devices! Mostly it's all trial & error (and a lot of learning) so you just need to practice but keep some basic stuff like that in mind!