I believe modifiers has to be applied before exporting, but that is something you can specify in the export settings, so you dont actually have to apply them within blender, its done for you during the exporting process.
Regarding one, or multiple objects - it does not matter. Use a system that works for you. I dont know if OBJ supports multiple objects, but even then, worst case scenario is it’ll merge it for you, so again it doesnt matter.
Im fairly certain FBX files do support it, however.
In regards to the materials and textures, that will also be sorted for you. Materials is a part of the export data, and if you want to use a texture, then you just wack it back onto the model in sketchfab, and it’ll take the UV coordinates of said object and reapply the texture correctly, automatically.