if your computer/device can handle it i'd recommend blender 3d
its typically more associated with game dev and the like but you can construct environments, buildings and other structures.
a thing to note about this or any other tool tho is acknowledging them as such and using them to build on. like no you may not know exactly what the inside of a spaceport looks like and no one knows what a witches house looks like save for whatever someone has imagined so for a lot of stuff the best you can do is construct a base and fudge the other details, using whats rendered as a staring point and adding your own style on top (like freemints mentions above)
there's also the fact that not everyone will be as invested in the details of the backgrounds and environments you include so dont spend too much time stressing every little detail. most readers (at least that i've noticed) are usually pretty passive when looking at page details unless they're the type to really care about that stuff so often times when including backgrounds they're usually limited to a few establishing shots and then whatever room or space is relevant to whichever scene (only ever really expanding on them if the characters are interacting with items or other elements within that space)
also seconding the point about making sure the references you use are genuinely free to use for commercial purposes as violating certain tos can get you in hot water with tapas or your ref sources