As @Okidoki said, when multiple objects with UVs get joined the UVs get joined also…that is normal. The next steps are what you would normally do to export to a game engine.
With the UVs mixed like this, select all in the UV space then select
UV > Average Islands Scale, and you get something like this…
With everything still selected select UV > Pack Islands…
and then you get…
Now you have a good UV that you can use to BAKE down the textures to a single image…
as shown here…
this allows you to export a joined object with just one material to your game engine…
Happy Blending!