I mean if you're editing the game's maps, that's inherently modifying the game (i.e. modding). If you don't want to use someone else's mod, you can certainly create your own! The wiki has a lot of good resources on the subject: https://stardewvalleywiki.com/Modding:Index
If you have additional questions, Modding Discussion & Creation would be a good spot to ask!
It's definitely something you can pick up on your own! Some mods are a lot more complex than others, but the wiki is a great starting point. Best of luck!I didn't know if it was something I could just find online, or if it was more in-depth and required some kind of college course.