Hi denizyenidunya,
We're glad you've reached out for help with this issue you're experiencing after sending videos from your Mac to iPhone using AirDrop. Let's get to the bottom of this together.
To start, please let us know more about what's going on. What's the exact version of iOS installed on your iPhone 12? This can be found under Settings > General > About.
It sounds like this is occurring with every video you send, although they play correctly on your Mac. Is this correct?
Does similar behavior occur when watching videos recorded directly on your iPhone? Have you had a chance to restart both your iPhone and Mac before attempting again to send these videos?
What happens if you record a video with your iPhone, send it to your Mac using AirDrop, and then send it back to your iPhone and test playback?
The following guide has recommended steps to restart your iPhone:
Restart your iPhone - Apple Support
And here's a guide for Mac:
Log out, sleep, wake, restart, or shut down your Mac - Apple Support
Have a great day!