Newbie question : where to put "advanced" page content?
IMHO it's easier to start with content, and add the design once the content structure is ready. Drupal tries to separate content and design as much as possible, so you can always change one and keep the other.

There are no mandatory modules, especially with Drupal 8 you can get very far without any additional modules.

Drupal stores the content of a page as "nodes". Nodes have a type, like "article" or "product". You can use the default node types that you get out-of-the-box, or define your own. Each node type has a number of fields, like a text fields, image fields, date fields, etc. You can enhance node types by adding additional fields. Drupal will take care of rendering those fields on a form, storing the content in the database, and displaying the content on the page. There is a lot more I could say about this, but I recommend that you read the documentation, especially the part about content structure.