How to interpret null-coalescing operators in c#

I have been given some code using nullable types, which are new to me (I learnt C# many years ago, in its infancy, and haven't kept up, sorry!)

The code I am trying to understand is

public readonly Trie?[] next = new Trie?[26];

and

var t = next[u] ??= new Trie();
t.insert(p[1..]);

To understand what's going on here, I have tried to rewrite this as

    if (next is null)
    {
         var t = new Trie();
         t.insert(p[1..]);
    }
    else
    {
         var t = next[u];

         if (t is null)
               t = new Trie();

         t.insert(p[1..]);
     }

but it doesn't return the same results when the code is run.

Question 1: is it possible to rewrite this nullable syntax in 'old-school' C#?

Question 2: if so, what's wrong with my interpretation?

If I can answer these, I will be able to start using these types in my own code

For given fragment

var t = next[u] ??= new Trie();
t.insert(p[1..]);

let's add parenthesis:

// check if next[u] is null and if it is, 
// create and assign new instance of Trie to it 
var t = (next[u] ??= new Trie());

t.insert(p[1..]);

Then we can translate it int good old style as

if (next[u] == null) 
  next[u] = new Trie();

var t = next[u];

t.insert(p[1..]);