Propping up double hung window
You really ought to repair the original sash mechanisms. There are many types but most are repairable, albeit with various degrees of difficulty. On the plus side, you will become an expert on how your windows operate.

There are various products made to solve your problem. For an easy solution on wood frame windows the simplest is the gull-wing shaped "hold-up spring", which should be available at any decent hardware store. It basically just sits between the sash and frame, exerting tension on the sash and holding it up yet allowing it to move when needed:

https://www.allaboutdoors.com/Spring-Quickie-Wood-Window-Sash-Wood-Sash-Hardware-Steel