Watched this tutorial and it seems interesting. As a use case - the Architect site I am building has a "Projects" section, and many projects are simple - so I've set up normal field types for the layout using project name, address, description, and gallery.
For Featured Projects which have much more detail than simple projects, it seems like I could use these additional flexible content fields to add in extra sections where she would explain more detail and images about different parts of the project, or a secondary gallery section?
Would I use a conditional field that she'd have to indicate at the beginning if this was a featured project, then the flexible content fields would be visible - whereas the simple projects, these would not show for her?
Unless you have different kinds of projects to showcase entirely, then Flexible Content might not be the answer.
ACF allows you to show and hide fields with conditional logic similar to something like Gravity Forms.
As an example, you could have a radio button at the top of the new Project screen that says "Featured Project?" with yes or no options and then set the extra fields that should appear to show up if that radio option is set to yes.
I don't mean to steer you away from Flexible Content, it's just that on your side you have to code it all manually in Oxygen. Depending on the number of sections and complexities of those, it might be a bit tricky.
You could also consider building Guternberg blocks and letting the client build the layouts that way.