“So basically, use the page builder for widgets and forget about Appearance > Widgets with the widget areas etc?”
Page builders are made to help you create complicated layout for page content if you are not too good with HTML.
Once you create the layout using the page builder, you put actual content in these layout blocks, either manually (images Text …), or using Widgets.
Widgets are preformatted content blocks. They were originally created as a standard WordPress tool to add content to Widget Areas, in Appearance > Widgets.
A widget area is just a piece of layout the theme provides to create your site basic layout, like Sidebars, header widget area, footer widget area etc. Widgets are a way to populate these areas with content.
The theme creates the first level of layout, header, content, footer… The page builder gives you a tool to create a layout inside the content portion of the theme layout.
Widgets can be used as preformatted content blocks to populates both of these