Using multiple materials on a object

I have a basic question on texturing objects. The problem is that I of course can apply different materials to particular faces in a modelling programme like Blender. However, I want to paint materials onto an object. Until now, I just scale the uvs of my object up to the best size of the texture but this doesn´t look very well.
When I download pbr textures from for example I don´t know how to paint these finished materials on to my objects. I know that there are some professional programmes like Substance but I wonder if there is a free option to to this. Do you know a solution for this?

usually this is done in an art tool, such as Maya or Blender, you can map materials onto selected faces or objects in different ways… flat projection (planar) , cube, cylindrical, spherical etc… then you can unwrap the uv’s to see how they look (polygons overlaid onto the texture, and you can edit/tweak them by hand. Then you re-export the object. typically .fbx format.

You can also do this in unity with a few tools that you can find in the asset store.
Try Pro-Builder… now comes free with unity, but there are others.

You can also use (for some cases) shaders to map procedurally, for example tri-planar mapping shaders… that map and blend from all 3 axis directions at once… good for terrain, rocks, asteroids… but comes at a performance cost. find some free or cheap shaders on asset store.

Polybrush might do it. Its in beta, but works for some people. Its on the asset store if you want to check it out.