come posso modificare la qualità delle registrazioni schermo in quicktime player

On what model Mac are you making these recordings? Identify the model of your Apple device - Apple Support

What is the version macOS on that computer? Find out which macOS your Mac is using - Apple Support

To reduce the size (and thereby the quality) of QuickTime screen recording movies, you would have to re-encode the existing movie at lower quality. Depending on how powerful the Mac is, that may take some time. HandBrake is a often recommended tool for re-encoding movies with different parameters.

For any movie (every source) this is true: file size = bitrate × duration. Duration is simple: shorter movie clips are smaller in size, and near-linearly. Bitrate is a combination of frame size, frame rate, and codec efficiency. While bitrate is often not exactly constant, it is quite predictable with averages. Most users want to keep frame rate as-is, or else you loose fluid motion. For codec efficiency, H.265 HEVC creates smaller movies than H.264 AVC — if the playback device is compatible.

Audio is very compact compared to video, so don’t worry about that too much, although you can save a bit there as well, if you must.

To give more concrete advise, please answer these:

1. How long (runtime) are your typical screen recoding movies?
2. What size in pixels (width×height) is your screen capture movie? Would scaling down (smaller frame size) be acceptable?
3. What size (in bytes) are your movies now? How small do you need them to be? Movie files are big by their nature. Don’t expect email attachment sizes.
4. Do you have a special use in mind for these recordings? That would help formulate a ‘best strategy’.

You can also explore other screen recording applications, to see what settings for quality those provide. It would be different, but not necessarily better. I have no advise for that, though. Maybe someone else here does?

If you upload to YouTube, then the format or file size doesn’t matter much. YouTube will accept many formats, and re-encode (convert) uploaded videos automatically to various resolution sizes and qualities.