The answer is it depends. What I would advise you to do is to have an app/Support folder where you put these classes that have custom behaviour, and then you use these classes inside your Controllers or Commands.

I would advise to experiment! Try different folders, and refactor when you feel like that approach is too complicated to maintain. That way you will understand the pros/cons of everything you tried and will give you good opinions when starting a new project.

Btw, to calculate the cube root, you simply do pow($number, 1/3); . No need to create a class for it 😅

As @juampi92 said, there are many ways. Popular way is to add app/Support/Math.php or app/Helpers/Math.php where you will define your methods. Then autoload the file in composer.json:

"autoload": {
        "psr-4": {
            "App\\": "app/",
            "Database\\Factories\\": "database/factories/",
            "Database\\Seeders\\": "database/seeders/"
        },
        "files": [
            "app/Support/Math.php"
        ]
    },

That way they will be available in all of your classes.

A more OOP approach is to define Traits. Again, you can create them in any directory you chose, I mainly use app/Traits, app/Models/Traits,... So, app/Traits/Math.php would look like:

<?php

namespace App\Traits;

trait Math
{
    protected function add(int $a, int $b): int
    {
        return $a + $b;
    }
}

Then your model would be:

</php

namespace App\Models;

use App\Traits\Math;

class Addition extends Model
{
    use Math; // Extend class with trait

    ...

    public function add() : int
    {
        return $this->add($this->a, $this->b);  // Method from trait
    }
}