How does whereHas work in Laravel?

Question

Laravel

How does whereHas work in Laravel in this example:

$orders = OrderModel::whereHas('User', function ($query) {
            if ($request->has('term')) {
                $query->where('LastName', 'like', $request->term);
            }
        });

How many queries will Laravel build if the condition is true, $query->where()and what query will it build otherwise?
And is it possible to dynamically create a query based on input data, something like this:

$query = "SELECT * FROM TABLE";

if ($request->data) {
  $query+= " lEFT JOIN TABLE 2 ON..."
}

Answer 1

How does whereHas work in Laravel?

Just whereHas:
If we add more with:
Great article

Answer 2

Instead of ->get() or ->first() you can call ->toSql() it will show the current query template

Answer 3

How many queries will Laravel build if the $query->where() condition is met, and what query will it build otherwise?

This can be easily found out from the database logs or the debug panel for Laravel.

And is it possible to dynamically create a query based on input data, something like this:

Maybe. And what prevents you from building such requests?