So for example you can use Excel online with Sharepoint/OneDrive (two different hosts btw) but you can also use many maany third party hosts.
Additionally third party tools can programmatically access the first party hosts (like sharepoint).
I don't like Microsoft-esque APIs and the company sure has issues here and there but I doubt you'd get the same level of data privacy with a startup (e.g. everything goes through privacy review, security review, devs can't access customer data, data is separated by region etc)