I think I have identified the error.

First I need to import the CSS part

symfony console importmap:req bootstrap/dist/css/bootstrap.min.css

And insert the import in asset/app.js

import 'bootstrap/dist/css/bootstrap.min.css';

Then I need to import the correct JS part, which is ..bundle.. and not ..min..

symfony console importmap:req bootstrap/dist/js/bootstrap.bundle.min.js

And insert the correct import in asset/app.js

import 'bootstrap/dist/js/bootstrap.bundle.min.js';

Now it seems everything is working correctly.