I need to call SharePoint REST API v1.0 endpoints, i.e.
The caller is a multitenant ASP.NET web app, not related in any way to the tenant of SharePoint server. In fact, I should be able to access any tenant that provides consent. Following instructions at https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/complete-basic-operations-using-sharepoint-rest-endpoints I need to obtain an OAuth 2.0 access token but the instructions for that process are missing.
Ideally I do not want to interact with SharePoint in any other way (ask admins for permissions, install add-ins, authorize my app etc.) - only call APIs (read-only) on behalf of a user I sign via Azure AD.
Technically I could instead work with MS Graph API and obtain tokens for that, but at the moment looking specifically for REST v1.0 API to remain compatible with on prem deployments of SharePoint