The best thing would be to watch tutorials/courses related to modeling and UVing in a specific modeling software of your choice. Unreal Engine can’t do that kind of fix, only modeling software can. I don’t know anything about Chief Architect, thus I have no idea of its possibilities or limitations. But there’s a lot of materials available, even for free, for Blender, 3DS Max, and Maya. Once you have a good understanding of topology, smoothing groups, UV’s, and optimization, it will become easier to spot problematic areas on your models.
Some of the most common issues are flipped normals, broken normals, N-gons, overlapping meshes, non-welded vertices, and messed UV’s (generally due to either poor unwrapping, or automatic unwrapping).
The reason why things are looking pretty in Blender is probably that their materials are solid colors.
Within Unreal Engine, it’s possible to see that their UV’s are messed up, as UE applied a UV checker on them. This happens because the UV’s are the ones responsible for mapping the textures onto their respective meshes (they tell which areas of the texture go to which areas of the mesh). And if the UV’s are messed, well, the textures will be messed too (as shown with a checker texture on UE).
I’m not a Blender user, so I can’t suggest any good tutorials to you. But you’ll probably find something by searching for terms like “modeling for Blender”, “ArchViz modeling for Blender”, or related things. Maybe some tutorials on game assets for Blender would be welcome too, as the game workflow requires more optimization.
Regarding the meshes being grouped by material, personally, I think this is annoying, as it makes editing the meshes a little harder. But this is definitely not the end of the world, and depending on your needs, maybe this is not even a thing. But is a good practice to keep doors and all other moving parts as individual meshes, with correctly placed pivot points.