From subquery syntax

Hi,

Since laravel 6 there is the ability to add From Subquery statements.

https://github.com/laravel/framework/pull/29602

The examples given for the syntax are this:

$query->select('name')
    ->from(function ($query) {
        $query->something();
    }, 'users')
    ->get();

DB::table(function ($query) {
    $query->something();
}, 'users')->get();

I'm very confused whats the difference between the two and how to implement it. Given I have the following postgres query to find consecutive date streaks when a given user does a session.

SELECT COUNT(*) streak, SUM(amount) streakAmount, MIN(date) startDate, MAX(date) endDate,
        dateMinusRow dateMinusRow
      FROM (
        SELECT 
          COUNT(*) amount,
       date_trunc('day', start) date,
            date_trunc('day', start) - INTERVAL '1' DAY * DENSE_RANK() OVER (ORDER BY date_trunc('day', start)) dateMinusRow
        FROM sessions
        WHERE user_id = ".$this->user->id."
        GROUP BY date_trunc('day', start)
      ) groupedDays
      GROUP BY dateMinusRow

I'm having trouble understanding how to add the surrounding query, I think I got the from() part down:

DB::table(function ($query) {
            $query->selectRaw("
            COUNT(*) amount,
            date_trunc('day', start) date,
            date_trunc('day', start) - INTERVAL '1' DAY * DENSE_RANK() OVER (ORDER BY date_trunc('day', start)) dateMinusRow"
            )->from('sessions')
            ->where('user_id', $this->user->id)
            ->groupByRaw("date_trunc('day', start)");
        }, 'groupedDays')

But I don't understand how I should put the top level select and groupBy clauses