Not sure how organisation works

On the subject of “Folders” in Lightroom, their counterpart in Lightroom Classic would be “Collection Sets” (found in the Collections panel. Although “Albums” in Lightroom sync to “Collections” in LrC and vice versa, the problem is that the cloud-based “Folders” do not sync with “Collection Sets”. So it’s necessary to manage those manually, i.e. if you have Folders containing Albums in Lr, and the Albums sync to LrC as Collections, in the LrC Collections panel you could then create Collection Sets with the same name as the Lr Folders, then drag the appropriate Collections into the Collection Sets to mimic the Lr organisation.

Unless you have a specific requirement to use LrC (e.g. for some of its features which don’t exist in LrD) you might be better advised to use LrD instead of LrC as suggested by ​@richardplondon. That way you have full integration of LrD with the other Lightroom apps, and you have the ability to minimise the local hard drive usage.

I suggest you look into the desktop app for Lightroom. That installs locally on the computer, but its whole focus is your cloud-held image library. It merely shows another view onto that. Ingesting new images therefore uploads these straight into that same place (cloud storage) and under the exact same organisational options (Albums principally, put into whatever hierarchy of Albums you want, but also keywording etc).

Lightroom Classic is different from Lightroom conceptually, practically and organisationally. Even keywording does not transfer quite perfectly and folder (in the sense of file system location) cannot do so at all. 

Each person should IMO commit to EITHER one paradigm OR the other as primary, and then  accept that any interactions or partnering between these two can only be quite superficial and secondary. 

Specifically: LrC will only upload a smart preview - not the full file - when you tell it to synchronise a given Collection (LrC’s name for an Album) up into your cloud storage. So that is all that the various Lr apps will then see for this photo. 

These two alternative products’ apparent similarities - notably the term “folder” (which does not at all mean the same thing in the two contexts) - can very easily trip us up. LrC and Lr  cannot be used interchangeably, nor even ‘partner’ without stripping off some intelligence in one or the other direction.

But Lr desktop app / Lr web app / Lr tablet app - these are fully integrated.

LrC gives you only limited ability to organize cloud storage. Each collection in LrC that you sync with the cloud is a form of organization of cloud storage, as these become albums in the cloud. And that’s the entire organization capability in the cloud, as far as I know. None of this applies to videos. None of this applies to folders.

LrC does not give you the ability to remove photos from your PC to save space.

If you really want to store photos in the cloud, you can use Lightroom (not Classic). I don’t know what organization features are in Lightroom (not Classic) other than albums, as I don’t use it.