Maintain folder hierarchy

I think you mean migrate from Lightroom Classic to Lightroom Desktop. Lightroom Desktop is the name for the cloud-based version (yes very confusing indeed!). It indeed does not maintain your folders. All your images will be stored in the cloud. There is no folder structure there. You can put images in albums which are similar to collections. So the trick is to drag your folder structure over to the collections part in Lightroom Classic before you do a migration (described here: https://feedback-readonly.photoshop.com/conversations/lightroom-desktop-cloudbased/lightroom-desktop-import-maintaining-folder-structure/5f5f45db4b561a3d42637c06). You will then end up with a album structure in the cloud version (i.e. Lightroom Desktop) that reflects your old folder structure.

You cannot partially migrate. It is all or nothing.

It is important to realize that the cloud version is basically a port of the mobile version of Lightroom back to the dekstop operating systems (hence the name desktop!). It misses many essential features that Lightroom Classic has. You can't print at all for example, you can't have virtual copies, you can't have hierarchical keywords, you can't do publishing services, sharing to social/other platforms is much harder than it needs to be, you can't selectively sync to the cloud (everything always has to be there) and many other limitations. It has one nice feature which is that your Library will look the same regardless of which computer or mobile platform you look at aand the cloud storage relieves pressure on your local hard disk(s). So really know what you are getting into before migrating. It is not the right solution for most people that come from the Classic world, that do a lot of social media, or that are high volume shooters.

You can only "Migrate" from a Lightroom-CLASSIC catalog. (Not Lightroom-Desktop, the mobile version on a PC or Mac)

And, No, you do not have Folders and Folder hierarchy.

Note:

Collections of your photos in Lightroom Classic automatically become albums in Lightroom. The following data in your Lightroom Classic catalog won’t be available in Lightroom after migration:

• Snapshots
• Custom metadata
• Creations: books, slideshows, and print/web creations
• Shares on Lightroom for the web
• Map-saved locations
• Folders and folder hierarchy
• Smart collections
• Develop history
• Color labels (migrated as keywords in Lightroom

https://helpx.adobe.com/lightroom-cc/using/migrate-to-lightroom-cc.html