A modern staircase you can combine differently and apply different materials to the defined surfaces areas. See the examples in Unreal, you simply pull other materials on the model's surfaces and get as many variations as you like.
The download includes the .fbx model with UVs assigned for 1 square meter per 1 UV zero to one. The different areas have their unique material IDs, so you can optionally assign different textures/materials to them...or make them all the same.
You can import the .fbx model in many Programs, like Unreal, Unity, 3D programs, CAD, ...
Pay what you want. You get the source model in .fbx format.