When rinsing between washes, just use water or also use conditioner and/or curl cream stuff?

It kinda goes by feel. You can use water if you're just trying to do a surface level clean or use conditioner if it's hard to run your fingers through it. Use shampoo when you're oily/greasy or dusty. If your hair breaks easily when wet, you can use dry shampoo to extend the time between washes so you can maintain length.

Also, how do you find what products work for your hair? Like, just trial and error? Or is there some kind of methodology i could follow?

Companies try to convince you that it's pure science that makes their stuff universally good, but really you just have to go by feel and appearance. A lot of companies, especially catering to white buyers, use silicones to create the impression of shiny healthy hair when really it's just a coating of plastic.

It really is a trial and error thing. Everyone is slightly different and so what works for you might not work for someone else. If you find something that works really well, do research on the ingredients list, as you might be able to find a cheaper off brand or just bottles/jars of the ingredients doing the heavy lifting.

For a general guide:

https://www.realsimple.com/beauty-fashion/hair/hair-care/curl-types