Paris to Glasgow on EasyJet, any hints?

Put simply, as you are not checking luggage, all you need do is go through security and make your way to the boarding gate.

On your way, you will encounter a passport check for exiting the Schengen. You choose the line for non-EU citizens ie Others, Rest of the World, Autre Pays. There may be country flags to make it obvious. That's about it. It's designed so you don't have to think. It's all part of the fun of travelling 😁

In Glasgow, you go through UK Border control and you're in (hopefully).

“ We are US citizens traveling from a Schengen country (France) to a non-Schengen country(Scotland). There is a checkpoint for that, separate from passport control or customs, right?”

I can’t help but feel that you’re overthinking this a bit and also that you may have been watching videos of people transferring at CDG, instead of departing.
When you arrive at CDG you follow the signs to your gate. On your way there you will automatically encounter security and passport control. There is no way you can accidentally bypass these.
Passport control is to get you out of the Schengen zone. As the previous poster has already told you, there will be flags to indicate which line to stand in.
Once you’ve gone thru passport control, you’re in the non-Schengen part of the airport and can continue on to your gate. That’s it, there is no other checkpoint to go thru.

Ramblin' On and Dutch Traveler have already explained that you go through passport control to leave France and enter the UK but nothing extra.

I wouldn't bother with being there three hours before the flight - if flying to the US, yes, but not for a flight to the UK. Two hours should be fine.

The other thing to know is that you need Terminal 2B at CDG for EasyJet. It's not enough to just know "Terminal 2," as each one of the "sub terminals" at CDG are actually completely different buildings.

One of the videos mentioned paying for concierge service in case you want to skip lines and speed up the process. That seemed far fetched. Is that a real thing?

It is a real thing but I would never pay for that. I have better things to use my money for on vacation.

“you go through passport control to leave France and enter the UK but nothing extra.”

Just to clarify and to make sure the OP doesn’t go looking for the UK passport control at CDG; at CDG you will only go thru passport control to leave France/Schengen. The passport control to enter the UK takes place at Glasgow Airport.

The process will be the same as for any international flight out of the US: luggage check if needed, passport control, then security.
If you are not checking luggage, 3 hours is excessive. 2 hours is plentiful. Unless maybe you're planning to travel on Saturday, May 4th, or Thursday, May 8th, when I suspect lots of people will be taking advantage of 8-9 May public holidays to travel.

Thanks for the prompt and helpful advice!

When you get to Scotland, you will be ushered to Passport Control. With a US passport, you can use the egates rather than have to talk to someone. (There will be people there to help.)

After Passport Control, you will walk through baggage claim and then choose a "door" to walkthrough--Green or Red. (It's not really a door but an archway.) If you have nothing to declare and most tourists don't, walk through the green "door." You may or may not see any customs officials. If they want to stop you, they will. But its doubtful.

Just to clarify and to make sure the OP doesn’t go looking for the UK passport control at CDG; at CDG you will only go thru passport control to leave France/Schengen. The passport control to enter the UK takes place at Glasgow Airport.

Indeed.

You are all so great! Thanks. Moving on to the next question…
Thanks to Frank II for the clarification. Yes, sadly we will not get to see England by train on the way to Scotland. But I’m hoping we can just do an England and Ireland trip within a year or so.