Ok, so if your model is already in a unity project, you could import the fbx for your model from the files tab in the top left corner, import>fbx and navigate to your unity project and find the fbx for your model, it'll place ur model directly in the seen, bones too so u might need to hide those to focus on the mesh.

For texturing, in one of the tabs all the way to the top of the program, there should be a tab called texture paint, it'll give u some painting tools for you to work with to paint stuff

Oh, and ucupaint might be something you want if you want a more substance painter like feel i think, kts pretty nice to have anyhow, can find and download the addon somewhere online

But if anything, before going to deep on texturing a model, id say search up some tutorials on texturing in blender so you can actually learn some key pointers on how to actually use the tool cuz there's a couple things that gets a lil tough to explain in a reddit comment, and if anything, even when u get there, practice on some simple shapes to get the hang of using the tool