Understanding Include().ThenInclude() in EF core - At levels beyond the first

I know that Include then include should be used when an entity span multiple tables using what called eager loading querying; but I don't fully understand is the difference between include and theninclude

my guess is Include() is to access the first layer only and ThenInclude() is to access the next layer ?? but if that's right how can we access two collection in third layer with the following pattern

a -> a.b->a.b.a
        ->a.b.b

     a.c-> a.c.a

and here's two scenarios and my attempt to solve it, can someone also check if i'm doing it right

in a scenario where I have Order class that contain

public ICollection<basket> items { set; get; }
public Address address { set; get; }

Basket include

[BindNever]
    public Product product { get; set; }

2- Is this right way to query it?

 public IEnumerable<Order> Orders
 {

     get {  return context.Orders.Include(o => o.address).Include(o => o.items).ThenInclude(p => p.product);

         }
  }

and if I have recursive association

section/topic/post/ where section has subsection section definition include so here a subsection of type section

        [BindNever]
        public ICollection<Thread> threads { set; get; }

        [BindNever]
        public ICollection<Section> subSections { set; get; }

and Thread entity include

[BindNever]
        public ICollection<Portfolio_Domain.Entities.Post> Posts { set; get; }

3- is this a correct way to use it?

    public IEnumerable<Section> Sections
            {
                get {  return context.Sections.Include(s=>s.SubSections.Where(ss=>ss.subSectionID==s.ID)).Include(t=>t.Threads).ThenInclude(p=> p.Posts);
         }
            }

Update: I found my solution to 2- and 3- but not 1- yet

for #1 Iwant to achieve something like context.a->include(b).theninclude(b.a, b.b).include(c).theninclude(c.a) more importantly a code that work for any level, a code that can work for the next level too like following context.a->include(b).theninclude(b.b).theninclude(b.b.a,b.b.b)

I'm not struggling with including multiple data from the first level, the problem i'm struggling with is to including multiple datatypes from third or fourth level I hope this explain it and sorry for my English but this is the best I can explain