Each user might be a member of multiple organisations, using multiple services through multiple accounts on multiple platforms with multiple devices, each of which could have multiple users.
An example might be a user working for more than one client, using both Microsoft 365 and Google Workspace for different situations, having more than one account in each service to provide for different needs, and using both windows and apple devices, so requiring multiple Apple, Microsoft and Google accounts, and some or all of the computers or mobile devices might be shared with other users.
There is no unified solution to manage this scenario, so the user should be assisted and enabled to manage their own devices.
Technical Support and IT Administration depends on the particular needs of the organisation and of the users.
Centralised management of users and devices is not necessarily appropriate for smaller organisations, although it would be best if all users were committed to using 2FA/MFA and disk encryption, for security.
Centralised IT management starts to correspond to the constraints of organisations of 100 or more users, at which point the user is probably a member of that single organisation which will dictate the way the user works.
At a smaller level, IT Support can be more individualised and personalised.