We start from your plans, because a footing's width, depth and steel depend on what the building weighs, the soil and the code, and that's not something to guess at. An online calculator is handy for figuring concrete once those sizes are set, but it shouldn't be what picks them. We dig the trenches down to firm soil and keep the bottoms clean, with no loose dirt or standing water, since loose material under a footing is what settles later. If a trench gets dug too deep or the bottom goes soft, the fix is an approved correction, not loose dirt shoveled back in. Then we set the forms and the rebar, the steel bars that hold the concrete together under load, and have it all ready before the inspector comes. When a budget gets tight, the trench and the steel are the easiest things to skimp on because they're buried once the concrete's in, and I'd rather trim somewhere else.
Footings often come up alongside other work. If an older foundation nearby has settled and needs lifting back into place, that's foundation raising, and I can look at both on the same visit. When a new footing has to run through an existing slab, patio or driveway, we use concrete cutting to open a clean, straight line instead of breaking it out by hand.