icloud
I see here (bold my own):
With Apple One, you can choose a subscription plan that includes 50GB, 200GB or 2TB of iCloud storage. If you need more iCloud storage, you can buy more for a total of up to 4TB.
This seems to mean for people with more than 4TB of data (e.g. someone with lots of family videos, or a hobby photographer), there's simply no way to store it all in iCloud (without creating separate dummy accounts). Is there any way?
For reference, Google Drive goes up to 30TB
It will depend on the root cause of your storage being full, so start by checking on Settings > General > Usage, and select Manage Storage from there to have a detailed view.
In my case, pictures are using a lot of space, and podcasts are just behind.
If you click on any of the items on the lists, you get more granular options
Here I clicked Music and I'm able to see which albums are using space.
If I slide "All music" to the left I and click Delete I can remove it from my iPhone from here, and recover 1,9 GB of space; of course you have to be cautious and be sure you have that music somewhere else (on your computer, on iTunes Match, or if you purchased it from iTunes Store, you can re download it when you need it).
If it is full of pictures and videos, you can activate iCloud Photo Library and store full versions on iCloud, keeping low resolution copies on your device; that will free up some space. If it is music, iTunes Match or another streaming service will allow you to delete music and download —or stream— on demand.
Other than that, check Whatsapp and iMessage which are huge space eaters —because of images and videos sent and received with those services; you can delete those images and videos if you have copies of what is worth saving.
iCloud will be very useful for backups and synchronized documents between your Apple devices, but your question indicates you hoped to somehow move some of your data from your device to the cloud, which is called selective syncing on other services (Google Drive, Dropbox); I couldn't find info on that —or similar— feature in iCloud, and I believe it's not possible.
I ended up phoning up iCloud support, and they were similarly baffled. They tried a load of things (none of which worked), and eventually they then asked me to delete the WhatsApp Documents & Data storage. Despite it saying that only 350MB was being used by WhatsApp, deleting this immediately freed up around 2.8GB (350MB plus the ~2.5GB of "lost" space) in iCloud.
Best Answer
Before Apple One introduced 4TB iCloud storage plans, the limit was 2TB. There was no way to increase the limit past that biggest tier, and likewise now with the 4TB tier — there's no tier beyond the largest tier.
This has been a regular complaint of iCloud. The 4TB tier could have been in response to the years of asking for more than 2TB, but of course now we're in the same situation and delaying the inevitable for those of us who store a lot of data in iCloud.