If you aren’t sure how to navigate and inspect the right things, the above posts should help. But if you are past that point and basically now wondering how you can “pin” a card to the inspector no matter where you edit, then the following might help, and is a slightly more advanced dive into how a Scrivener project can be configured.
First, have a look at the feature described in §13.1.1, Locking the Inspector, in the user manual PDF. It’s probably not quite like “pinning”, in the sense you’re describing your query.The way it works is by locking the inspector to only work on one split or another. It doesn’t look like you are splitting the editor, so there’s no harm in trying out the use of splits for the one specific purpose.
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Use View ▸ Editor Layout ▸ Split Horizontally.
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Click into the top half of the editor, and select what represents your chapter, or wherever the notes and synopsis are, and turn off the current view mode (click the button that is highlighted again). In your case, you should only be seeing the epigraph text.
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Shrink the view down as small as it will go, or whatever you prefer, and follow the instructions to lock the inspector to the top split.
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Optional: use the View ▸ Binder Selection Affects ▸ Both Editors setting.
Now with that optional setting, whenever you click on a chapter, the text of it itself will load into the top split, the inspector will stay locked to it, and the full scrivenings mode text of that group will load into the bottom split.
Without the setting, clicking in the binder will impact whichever split is active, giving you more control over what you are inspecting, and what you are editing, in the top and bottom splits respectively.
If you find that useful, consider using Window ▸ Layouts ▸ Manage Layouts, and clicking the + button to add this arrangement as a saved layout you can return to in the future with a single click (well, you would still need to lock the inspector after using it, as we felt that tool was too specifically used and potentially confusing, for something like this).
@blunoz : I probably should move the epigraph to a separate file at the start of each chapter?
Yeah, that’s not a bad approach! Here is a little tip for doing that, which in fact addresses introductory images at the beginning of the chapter, but with an epigraph it’s really the same kind of thing at the level we’re talking about. Maybe don’t rabbit hole all of that at once, just know it’s quite fine to separate components of your chapter out into separate outline pieces, either by kind or by narrative purpose.
But there is no harm in using the folder text area for that kind of stuff too. Arguably it’s a very good place for preamble material that isn’t really a part of the main content of the chapter, but of the whole chapter. All elements of the draft can have text for a reason. It’ll be something you’d need to tweak your compile settings for, once you get to that point. There are factory presets that print both something’s title as a heading and its text content, which is a good setting when using “folder” text for stuff like epigraphs, “What you’ll learn in this chapter”, pictures, and so on.
Really it’s up to you. The software imposes few limits, so that you can use the outline and other inspector tools to your maximum advantage. Speaking of thinking outside of the box though, consider the implications of what I said above, by default whatever you type into a folder’s text area won’t export! That makes it a good choice for chapter notes. Look at that split set up… you’ve got a place right at the top of your chapter text, that stays fixed as you scroll, where you can type notes. It might be more straightforward than locking the inspector. You don’t even need the fancy dual-click split setup at that point, as Scrivenings mode by natural includes the group text at the very top. You can simply split the editor and keep the top half scrolled to the top as you work in the bottom half.
Again, more options… 
@GoalieDad : I assume on the Mac based on Inspector Icons. you should address in the Mac section.
For very fundamental question about how the software works, it is encouraged to not divide such queries into platform specific discussions. Those categories are better reserved for questions that have no actual answer from other platforms.
