Unity mouselook + WASD + camera.

WASDMoving.cs

// Based on work of FatiguedArtist from https://forum.unity.com/threads/a-free-simple-smooth-mouselook.73117/
// Added simple WASD controlling, which can be enabled by pressing right mouse button.
// Deleted comments and some fields. 

using UnityEngine;

public class WASDMoving : MonoBehaviour
{
    public Vector2 clampInDegrees = new Vector2(360, 180);
    public Vector2 sensitivity = new Vector2(2, 2);
    public Vector2 smoothing = new Vector2(3, 3);
    
    private Vector2 mouseAbsolute;
    private Vector2 smoothMouse;
    private Vector2 targetDirection;

    private void Start()
    {
        targetDirection = transform.localRotation.eulerAngles;
    }

    private void Update()
    {
        if (!Input.GetMouseButton(1)) return;
        
        // Mouse look
        var targetOrientation = Quaternion.Euler(targetDirection);

        var mouseDelta = new Vector2(Input.GetAxisRaw("Mouse X"), Input.GetAxisRaw("Mouse Y"));
        mouseDelta = Vector2.Scale(mouseDelta, new Vector2(sensitivity.x * smoothing.x, sensitivity.y * smoothing.y));

        smoothMouse.x = Mathf.Lerp(smoothMouse.x, mouseDelta.x, 1f / smoothing.x);
        smoothMouse.y = Mathf.Lerp(smoothMouse.y, mouseDelta.y, 1f / smoothing.y);

        mouseAbsolute += smoothMouse;

        if (clampInDegrees.x < 360)
            mouseAbsolute.x = Mathf.Clamp(mouseAbsolute.x, -clampInDegrees.x * 0.5f, clampInDegrees.x * 0.5f);

        if (clampInDegrees.y < 360)
            mouseAbsolute.y = Mathf.Clamp(mouseAbsolute.y, -clampInDegrees.y * 0.5f, clampInDegrees.y * 0.5f);

        transform.localRotation = Quaternion.AngleAxis(-mouseAbsolute.y, targetOrientation * Vector3.right) * targetOrientation;

        var yRotation = Quaternion.AngleAxis(mouseAbsolute.x, transform.InverseTransformDirection(Vector3.up));
        transform.localRotation *= yRotation;

        // WASD
        var isShiftPressed = Input.GetKey(KeyCode.LeftShift);

        var x = Input.GetAxis("Horizontal") * Time.deltaTime * 3f;
        var z = Input.GetAxis("Vertical") * Time.deltaTime * 3f;

        transform.Translate(isShiftPressed ? x * 3 : x, 0, isShiftPressed ? z * 3 : z);
    }
}