I have added a new field(tertiary_user_field) in the database table admin_user using the following.
1)Vendor/Module/etc/db_schema.xml

2)Vendor/Module/etc/adminhtml/di.xml

3)Vendor/Module/Plugin/Block/Adminhtml/User/Edit/Tab/Main.php

OUTPUT:

I have added a new field(tertiary_user_field) in the database table admin_user using the following.
1)Vendor/Module/etc/db_schema.xml

2)Vendor/Module/etc/adminhtml/di.xml

3)Vendor/Module/Plugin/Block/Adminhtml/User/Edit/Tab/Main.php

OUTPUT:
