using System;
using System.Linq;
using System.Text.Json;

string[][] teams = {
    new[] {"Alex", "Mark"}, 
    new[] {"Max", "Tom", "Ania"}
};

//first method
var names1 = teams
    .SelectMany(team => team)
    .Where(name => name.Contains("M"))
    .Select(name => new {
        Name = name, name.Length
});
//names1 is {{Name = "Mark", Length = 4},
//          {Name = "Max", Length = 3}}

Console.WriteLine(JsonSerializer.Serialize(names1));

//second method
var names2 = from team in teams
             from name in team
             where name.Contains("M")
             select new {
                 Name = name, name.Length
             };
//names2 is {{Name = Mark", Length = 4},
//          {Name = "Max", Length = 3}}  

Console.WriteLine(JsonSerializer.Serialize(names2));