Someone, somewhere, wrote or typed a document with system details. Whether the owner/manager gets that is up to the job/bid and requirements set forth at the beginning. If someone is required to hand over the design at some point in the project and they don't, it's likely that this will be the last job they do with that designer/archi/builder/client. If the project is large, it's very unlikely that someone thought about the IT design for a few minutes, offered a price for the work, supplies, equipment, setup and instruction and just did everything from memory. If it took a while to get approval to start working after the bid, someone should have been looking at some kind of document, to make sure they can still do the job at that price.
One reason to NOT leave a copy is: people take them. If it's just to see how someone else does this work, that's one thing but it could also be used as a template by a competitor. Another reason: people move things and don't put them back where they were found, so it may ne "lost" at some point.
Any documents should be handed to the owner of a house/small business or the facilities manager of a large plant but a master file for each client makes replacing these very easy.
My mechanic told me, "I couldn't repair your brakes, so I made your horn louder."