First, there are two basic ways of importing a mesh (or collection of meshes) in Unreal: the import asset (either by dragging your FBX file into the Unreal Content Browser or by clicking “import” from the content browser/ drawer top menu) or the Import into Level (from File > import into Level).
I personally never use the import into Level and always prefer to import each FBX as a separated mesh & combine them in Unreal. But let’s see what exactly are those two options:
import asset: it allows you to import your mesh either as a single asset (when checking “Combine Meshes” or when you only exported a single mesh) or as multiple assets. In Unreal, It’s called a Static Mesh when it only contains geometry data & materials, and a Skeletal Mesh when it also has bones or Blend Shapes/ Morph Targets, mostly for various animations.
Import into Level: from the FBX, it creates a Blueprint & keeps the original file structure. Each mesh inside the Blueprint is acting as a separated Static Mesh.
Like in Unity, you can drag the newly created Blueprint into your Viewport and get the whole mesh in a single part.
From there, you can even use the Graph Editor to add some small visual scripts, for example to animate a separated mesh (like opening/ closing the door). Beware, though, in your content folder, the Blueprint will be a single & separated file, and Meshes are NOT nested into the Blueprint file like in Unity (but you have an asset filter in Unreal to quickly see & drag your Blueprints into the viewport). They are only displayed when you open the Blueprint itself.
Thus, the Unreal Import into Level way is a bit similar to how Unity is importing FBX files.
In fact, each imported FBX in Unity is automatically keeping the file hierarchy and, when you drag it on the Hierarchy Window or Scene View of your Unity project, it’s instantiated as a prefab, although it’s more a pseudo-prefab with no ability to modify it (it is recognized as a prefab when you check with GetPrefabType, but you can’t change anything inside it).
Now, in Unreal, although Prefabs and Blueprints have major differences, if you want to keep things simple, the Unreal Blueprint is still the closest form to a Unity’s Prefab. The only difference with the import into level is that it creates a real Blueprint, while Unity is making an unchangeable pseudo prefab.
However, both in Unity and Unreal, importing a very large amount of meshes is definitely not the best way to do it, mainly because of performance issue (& also to keep a simple and understandable Mesh structure). So you need to keep the smallest amount of meshes & materials.
See this even in Unity’s doc:
Merge your Meshes together as much as possible. They should share Materials and Textures as much as possible. This has a huge performance benefit.
Now, from what I understand, you used a house generator app. I always make things from scratch & I personally never use the Import into Level feature in Unreal.
But what I’ll do in that case entirely depend upon how you will use that mesh. If you need to reuse some parts of the house (like doors, windows, walls and so on), then I will import the full FBX into Blender (or any full-fledged DCC), export each reusable part individually and put them together in Unreal, while making a Blueprint when I need to have some interactions (like a Blueprint for the door).
Otherwise, if you just need to display (render) this house for a simple showcase/ visualization project and don’t have to use any interaction (like opening doors or windows), I would only import the FBX as a single combined Mesh, create Materials and that’s it.
At last, you still have a third option, the Datasmith plugin, but from what I know, it doesn’t seem to work properly on the Early Access version of UE5, you need to wait for the final 5.0 version.
Hope it helps