I’m confused with the 'Next-Action’ list. Where do I put that in?
It happens naturally as you tag your actions. You will view your next action lists by tag. Tags in Omnifocus are how it now handles setting contexts.
There isn't ever a single "Next Action List" abut rather a series of lists based on the tools, people, locations, mindset or time you need to complete that task. The context in which you can do the work. GTD book and set-up guides have a starter set of contexts but the power of GTD comes when you create and use contexts that are meaningful to you. My Omnifocus system has 42 different "Next action" lists that correspond to my contexts.
I struggle with single simple tasks, which are not part of a bigger project.
How do you handle that? Putting these in projects, doesn't seem to be a good fit, because projects themselves are tasks and projects like 'Home' aren't tasks.
To answer the first part fo your question there is a concept in Omnifocus of a Single Action List. That is a special type of project that can contain those one-off only take 1 action items. I have 2 one for 1 off actions like a recurring action to "read one of my inspirational quotes" that happens daily. (I'm trying to build that into a habit which is why it's in my Omnifocus system) and one for Errands, that I used to remind me of all the stores I needed to go to when we went shopping. Even before COVID we only did major shopping once a month so I would add something to my shopping list and at the same time add a go to that store in my errands list so I wouldn't forget it.
projects like 'Home' aren't tasks.
Home is not a project. Home may be an area of focus. What do you have to do at home? What's considered done with home?
Here are some examples of projects I have in my AOF of Household Management
Rip new audio CDs into Music app for car player
Update my books owned lists (I have 3 one for non-fiction, one for fiction and one for kindle books)
Update map software in the car
Clean out the west bay of the garage
Omnifocus is a very full featured and complex app. It has a lot of power and it can be overwhelming at first. I'd suggest starting slowly and then implement more features into your personal GTD system as you find out you need them.