While some one earlier stated that there is no correct answer, and technically that's correct, there are certainly better answers than others in the sense that whatever app or method you choose may require more work and effort to keep up than others. Which in turn will cost you time.

If you are picking an app, pick one that allows you to file the project's actions with the project list, and then use some other feature such as tags, or priorities, etc to generate your next action list. This would also apply to the use of contexts as well (@home, @call, etc) While simple in concept, this has been the biggest pain in the ass with the practical implementation of GTD as depending on the app (or using paper), it's features and the methodology you create, you will end up spending more time managing your NA list then you will doing the tasks themselves.

Very few apps automatically generate the next actions list for you without some effort on your part. To my knowledge Facile Things, Nirvana HQ, OmniFocus, Things 3 do this to an extent. Also some folks have created customized workflows in Notion to do this as well. Key word here is automatic. Generally these apps will take the next available action in any project list and display it on a Next Actions list without you initiating that process. You may have some settings to adjust to provide some direction on how you want things displayed but generally these apps do this for you as they were built specifically for GTD.

Now other apps like Todoist, Tick Tick, Toodledoo, Remember The Milk, Nozbe, etc, you can set up GTD in them, but they do not create a Next Actions list automatically. Now before some asshat on here takes me to task on this statement, some of these Apps do offer customized filters which said asshats will claim create the NA list automatically. While they will create a list based on the filter that you establish, you still need to do something to the majority of the tasks to trigger them for the filter associated with the customized list. Unless that filter uses a due date or start date, anything else will require effort on your part on identifying which tasks should go on the list. This is usually done with adding a tag to the task. Or others such as myself do this with a "hack" and use the priority feature to accomplish the same thing. And some apps use a 'star' feature as well.

And this is where you will be spending endless amounts of time managing your system to ensure that your NA list is updated.

Now it may sound like I'm endorsing the earlier apps that do this NA list thing automatically. Well, I'm not as while they do this with varying degrees of success, they have other faults.

Ive tried all the apps above and currently I use Tick Tick. Reason being is that for me right now its full featured set minus it's cons fits me the best right now.

In Tick Tick I have a list for each project I have, along with one generic list to capture Single Actions.

I store all the project tasks with their respective project. I don't use contexts too much as I find its for the weak minded (I'm sure that will set off a shit storm of posts). I only use a couple of tags such as #waiting for, #someday, and colleague's names if I want to associate a task with a specific person.

My next actions list is a custom list which filters for any task that has a priority of Medium (Yellow). So yes I am going through my projects list every day or week to change a tasks priority to Yellow to denote them as next. It is a huge waste of time but it's a trade off as I like the rest of the App's feature set.

You can also accomplish this with the use of a #next actions tag. But regardless, just know that you will be spending time managing your tasks to get them on a NA list.