WordPress page content editors are not compatible with each other. If you use a specific page editor to create your page content, you can not edit it with a different page editor.
Demo content ↓ Page builders ↓ Theme builders
Theme demo content
Most of my theme demo content pages (and thus theme demo websites) are built using native WordPress editor. (If there is an exception to this, it is clearly stated in theme documentation and demo content required plugins list.)
Using the WordPress native block editor for theme demo content allows everybody to edit demo pages out of the box without the need to install additional 3rd party editor plugins (or page builder plugins).
Page builders
I understand customers may prefer using a different editor, such as one of the page builder plugins. These plugins are not compatible with default WordPress editor, nor with each other. So, you can not interchangeably switch between those editors.
Please understand this is not an issue with a theme though. My themes work with any editor and page builder plugin (such as Beaver Builder or Elementor), but demo content is always provided in a specific editor, usually the WordPress default one.
You are free to use your preferred page builder, but you will most likely need to built your pages from scratch. It is not possible to edit theme demo content when the content was not built in your page builder plugin.
Also, please understand it is not possible for me as a theme author to provide demo content for every page builder a customer may use.
But even though you can not edit the theme demo content with your page builder plugin, most of such plugins contain a library of pre-built page layouts for you. Or you can even use additional library plugin for that, such as Envato Elements or Starter Templates.
Tip: If you already have activated a page builder for the page and thus lost the ability to edit it’s content with original editor, you can always revert back with WordPress revisions (check out thorough tutorial).
Theme builders
Currently, WordPress allows you to edit all sections and layouts of a theme with block-based Site Editor. The most recent WebMan Design themes are compatible with this WordPress-native functionality.
Customers also prefer using theme builder plugins such as Beaver Themer and Elementor Pro Theme Builder. They will provide you with the same functionality as WordPress Site Editor: overriding all theme sections and layouts.
Again, using such theme builder editor plugin is not compatible with native WordPress Site Editor.
WebMan Design provides some themes that are compatible with theme builder plugins. However, you might need to install additional compatibility layer plugin (check your theme documentation):