Access the mouse with:

Mouse.current

Buttons:

Mouse.current.leftButton
Mouse.current.rightButton
Mouse.current.middleButton

Position:

Vector2 position = Mouse.current.position.ReadValue();

Delta:

Vector2 delta = Mouse.current.delta.ReadValue();

Scroll:

float scroll = Mouse.current.scroll.ReadValue().y;

Lock cursor:

Cursor.lockState = CursorLockMode.Locked;
Cursor.visible = false;