What "SelectMany", "For statements" and "from / for expressions" have in common

forComprehension.cs

var l1 = new[]
{
    new
    {
        name = "Jana",
        projects = new[] {"p1", "p3"}
    },
    new
    {
        name = "Ronald",
        projects = new[] {"p1", "p2"}
    },
};

var res1 = l1
    .SelectMany(person => person.projects
        .SelectMany(project => project.ToCharArray())
        .Select(char.ToUpperInvariant))
    .ToList();

var res2 = new List<char>();
foreach (var person in l1)
foreach (var project in person.projects)
foreach (var c in project.ToCharArray().Select(char.ToUpperInvariant))
      res2.Add(c);

var res3 =
    from person in l1
    from project in person.projects
    from c in project.ToCharArray().Select(char.ToUpperInvariant)
    select c;