Hi Zoe!

You can edit textures or geometry in Blender to fit your project, as long as you stick to the license. Most allow mods for your work, but redistributing edited models might be a no-go.

Check the specific model’s page for details or email us if unsure. Generally it's a good idea to check a model details page. Just below the price / download button there are types of included files land more details, if you scroll down in the Model Details tab where in the additional information you have file formats listed, i.e. FBX, TIFF (for textures), Blender, glt, glb, etc.  Also size of the files and specific files info under tags. In regards to source - that may vary per model. Anyway you can always use FBX to import and edit it for your needs. Make sure not to mess up UVs though unless you want to re-do them  but I presum you know what you're doing.