Need more details about how to sync with external database

Our organisation use an MS Access database for the membership. That database is most accurate regarding post- and email adress and phone number. I want to integrate that register with Nextcloud. I want 2-way sync of these data. The user_sql app seems to be what I want.
I do not understand the information given about the use of the app. There is information how to connect to the local mysql-database but nothing about the external database. Many supported options are described. Where can I choose these options? Do they occur in a user-interface?
I have started to configure with connecting the local mysql-database, but dont know what to do then. How can I point to the external database? How can I define the fields that I want to sync?
I need more information.

The membership of what? How do users authenticate against that?

If you don’t integrate with an SQL server it’s going to be difficult.

OK. Tell me how to authenticate against that database.
All 400 members in the organisation are registered in an MS Access database. Among the variables are name, e-mail address, group membership, sex, birth date, post address and other things.
We want to use NC for the collaboration in groups. About 50 of the members may use NC.
I have imported the 400 members into the Contact list so for the moment all values are up to date.

In the future the users of NC may change values in the Contact list. Group membership, e-mail adress are the most important. And the administrators in the organisation may do changes to the Access database. Add new members for instance. I want these changes to sync 2 ways.

user_sql is described “Use external database as a source for Nextcloud users and groups” and there are options to sync on e-mail and other things. But I only see information to connect to the nextcloud mysql database. Where can I give the connection information for the external database? If needed I can import the MS Access database to a mysql database and probably arrange the sync between them, but right now I do not understand the instructions for the user_sql app.

How do users authenticate against that at the moment? If they don’t, because it is an MS Access database and not an SQL database, then the process is not going to be straightforward.

Your data needs to be in a format accessible by the plugin, so if you look here, that is MySQL (probably MariaDB) and PostgreSQL. If your data is in an MS Access document then it’s not going to work.