In the reunion, it is assumed that the power of the crimson witch comes from the spiritual gem. Since the spiritual gem has been destroyed, why can she still use magic?

Six infinite gems are the embodiment of Marvel's cosmic laws, so they can have magical abilities! And some people (such as Thanos) can't use infinite gems directly, so they can only use infinite gloves to use infinite gems!

Since the infinite gem is the embodiment of the rules of the universe, it is roughly equivalent to a valve, which can extract the power of the universe, and the energy of the rules of the universe is endless and will live and die with the universe! Infinite gems (valves) can also exist at different times. So the destruction of infinite gems will not affect the rules of the universe!

Thus, the scarlet witch came into contact with the law of the universe-chaotic magic through the valve of the spiritual gem. With chaotic magic, there is no need for a valve with a spiritual gem.