So you wanna modify Vintage Story. First, you need to decide on what type of mod you want to make. There are three types of mods in Vintage Story. There's theme packs, content, and code.
Theme pack mods only add png files also known as textures and sound files. They can change the look of blocks, items, and even entities. What they can't do is add new stuff to the game. Theme packs also can't change the shapes of blocks and creatures since that would involve editing json files.
Content mods in addition to everything them packs and do can also add and edit json files to the game. This gives content mods the power to add new blocks, items, recipes, and much more. What limits them is adding new behaviors and mechanics to Vintage Story. So while content mods can squirrels for example, they can't do something unique to them such as randomly spawn in oak trees.
Code mods don't have any limits. They can do anything but are a much higher learning curve requiring knowledge of C# coding and the game API.
Protip Hitting Ctrl M in game will allow you to set macros. Macros will allow you to do a set of commands with a button press. Setting a macro for `.reload textures` and `.reload shapes` can save you time when working with content and theme pack editing by not having to restart the game each time you make a change. I also use macros for changing game modes to make it quicker.
Before making a mod you should get very familiar with the files setup for Vintage Story because that is how the game will expect you to have your files setup. This leads us to assets.