From which source does the SharePoint People Picker field gets users if I search a user in that field? Is it User Profile Service or Active Directory or is it UserInfo List table of SharePoint stored within database? What about SharePoint Online users then, from where the users are fetched?
I think In SharePoint on prem, when using NTLM the users are fetched from the hidden users list or AD. When using ADFS you have to do some more work for the people picker to work. In SharePoint online the users are fetched from AAD and site users