The option Use existing solution allow users to select a specific solution for this app. Users can create a whole new design from scratch by not checking check box of use existing solution.
Reference:
https://www.inogic.com/blog/2019/02/create-model-driven-app-cds-environment/

With the Common Data Service connector, you can create Power Automate flows that are initiated by create and update events within Dataverse. Additionally, you can perform create, update, retrieve, and delete actions on records within Dataverse.
You can use scopes to determine if your flow runs if you create a new record, if a new record is created by a user within your business unit, or if a new record is created by any user in your organization.
Reference:
https://docs.microsoft.com/en-us/power-automate/connection-cds

Liquid is an open-source template language integrated into portals. It can be used to add dynamic content to pages, and to create a wide variety of custom templates. Using Liquid, you can:
Add dynamic content directly to the Copy field of a webpage or the content of a content snippet.
Store source content by using web templates, entirely through configuration within Power Apps, for use throughout the Power Apps portals content management system.
Render a website header and primary navigation bar, entirely through configuration within Power Apps.
Note: page is one of the available liquid objects.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/portals/liquid/liquid-overview
https://docs.microsoft.com/en-us/powerapps/maker/portals/liquid/liquid-objects#page