Can you expand on this? Like people who have full time jobs in perhaps a different field but do research on the side using the facilities/resources of a university? If so how does one slide into a gig like that?
Without a plan, per se, I nurtured a burgeoning personal research interest that was well outside my field. The initial hobby was very rudimentary but it gave me enough feedback and fuel that over a number of years I built more advanced software and hardware to investigate it more scientifically. Eventually, having reached the limits of the data I had access to, I reached out to a local university to start a collaboration.
If you already have an idea, I think, as long as you're sufficiently interested in actually doing research, you'll be able to find some professor that is as well. On the flip side, I know for a fact that most professors have more research ideas than students, so they can certainly provide you with things to investigate.
As a bonus, for the professor, since this is a side gig for you, there isn't an cost for them (or much less of one). Access to facilities and resources depend on the university. But I was certainly surprised by what I was given access to in the service of doing the research.
Let's put it this way. If you can write a (1) good grant proposal with a good chance of getting funded you are required these days to (2) get a professor to be the principal investigator if you want to spend it and I'd say (1) is much harder than (2). Not like it is easy, but it is possible.