No. Experience Builder will not replace or obsolete Web AppBuilder. Both builders will be running and maintained in parallel. Web AppBuilder will continue to be available.

No. You need to re-configure apps or rewrite custom widgets created with Web AppBuilder if you choose to move them to Experience Builder.

Create the proxy item by following the steps in the "Authorize the layer" section of the Use Living Atlas subscriber content in public maps and apps blog article.

Yes. You can generate a Web Experience Template or duplicate the Web Experience item as a starting point for creating similar web experiences. To create a template, click the More options button on the builder toolbar and choose Generate template. To create a copy of an experience or template, click the More options button on the item card in the respective gallery and choose Duplicate.

The Experience Builder team would love to hear from you about the app. You can provide feedback through the GeoNet ArcGIS Experience Builder community page. If you have comments about the documentation, use the feedback link on the topic page. (Contact Esri Technical Support if you need assistance.)