You can engineer a module in one ship at an Engineer base, as you know.
Let's say Long Range on an FSD, a common one, on a size 4 FSD.
That module can be used by many different ships. There is module storage at stations, and transfer mechanics to move it where you are now. Any ship that can install a size 4 or bigger can use that same module. Be careful not to Sell it by mistake!
Replace it with a cheap placeholder module when you want to store it.
You can also pin the blueprint at the Engineer, and use Remote Workshop at any station to upgrade another FSD ( in this example) module. Pin mods you are likely to use a lot.
It is much cheaper to transfer a couple modules than a whole ship. I store 3 FSD sizes 3, 4 and 5 so anytime I buy a ship and want to engineer it, there is a FSD module to start the trips.