Ground detection, mouse look, and jump

PlayerControllerWithPhysics.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerControllerWithPhysics : MonoBehaviour
{
    private Rigidbody rb;
    private float horizontal;
    private float vertical;
    public float Speed = 5;
    private float colliderSize;
    private bool jump;

    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody>();
        colliderSize = GetComponent<BoxCollider>().bounds.extents.y;
        Debug.Log(colliderSize);
    }

    // Update is called once per frame
    void Update()
    {
        //Read input
        horizontal = Input.GetAxis("Horizontal");
        vertical = Input.GetAxis("Vertical");
        if (Input.GetButtonDown("Jump"))
        {
            jump = true;
        }

    }

    void FixedUpdate()
    {
        //Move an object with physics
        //rb.velocity = new Vector3(1 * horizontal, rb.velocity.y, 1 * vertical);

        //No speed, no gravity (almost no gravity)
        //rb.velocity = transform.TransformDirection(horizontal, 0, vertical)

        //With speed, y gravity is overridden to be almost 0
        //rb.velocity = transform.TransformDirection(horizontal, 0, vertical) * Speed;

        //With speed and gravity in the y axis (ie normal gravity)
        rb.velocity = transform.TransformDirection(horizontal, 0, vertical) * Speed + (new Vector3(0, rb.velocity.y, 0));

        if (jump && IsGrounded())
        {
            rb.AddForce(0, 4, 0, ForceMode.Impulse);
            jump = false;
        }

        var rotationSpeed = 6f;
        //Method 1
        // based off of horizontal
        //var euler = transform.localEulerAngles;
        //euler.y += horizontal * rotationSpeed;
        //rb.rotation = Quaternion.Euler(euler);

        //Method 2
        //based off of mouse look
        rb.rotation = Quaternion.Euler(rb.rotation.eulerAngles + new Vector3(0f, rotationSpeed * Input.GetAxis("Mouse X"), 0f));

    }

    bool IsGrounded()
    {
        return Physics.Raycast(transform.position, Vector3.down, colliderSize + 0.1f);
    }

    private void OnTriggerEnter(Collider other)
    {
        //Destroy(other.gameObject);
    }
}