sharepoint is its own thing. if you use sharepoint, then the question would be how to access sharepoint files
'files' imply a specific storage process. Azure supports cloud file stores and you can access your files via standard file processing if you are running current operating systems. I am guessing that AWS may support this too. The downside to this is the cost is higher than the more typical storage of files in blob storage.
blob/block storage is what most people think of when talking about "cloud" files. To use this you need to either write your api access to the blob storage, or use software to emulate a disk drive.
for the api, every provider has its own api to access them (azure, aws, dropbox, etc). There have been several elevate presentations on this in the past several years - specifically azure blob storage and dropbox
for software, there are several products available. i've used cloudberry drive in the past, there are many others