Seeds that get planted out early (broccoli, kale, etc) are easily started in the little seed starting trays that are sold at garden centers. But, at least in our area, plants like squash, pumpkin, green peppers, etc. get planted out quite a while after they are started and grow quite large before they are ready to plant out. There is no way they would have enough dirt and space in a seed starting tray to grow big enough to transplant.
So, this is my simple solution: plastic disposable drinking cups.
I start my seeds in them, label them, water carefully, set along a window sill, and rotate as needed.
As I plant the seeds out in the garden, if the cup happens to break or needs broken in order to get the plant out safely, that isn't the end of the world. But, most of the cups can be saved for next year when they will be used to start the same seeds as they are labeled!
Super simple and super frugal way to start seeds.