When editing a Elements Connect field connected to URL datasource with OAuth2, what authorisation will my JIRA user have on the remote platform?

When you configure a datasource with OAuth2 in Elements Connect administration, you grant Elements Connect access on your own behalf. This access will be shared by everyone using a Elements Connect field on your JIRA instance.

Every user will have access to the same set of data. Of course, depending on the target system, you can have the possibility to configure the access scope that you gave to Elements Connect.