Reducing 'System Data' storage on MacBook Air

Have a look at > Free up storage space on Mac - Apple Support

I very much agree with @TheLitttes approach

Part 1 of 2

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

Always make a Time Machine Backup  before  proceeding 

It is also suggest to  Only Migrating the User Account and nothing more.

Emphasis on User Account Only

Reinstall the Needed and not the Wanted applications from the Apple Apps Store or Directly from the Developers 

Part 2 of 2

Generally when this issue is discovered by the User ( you in this case )

It is because the Internal Drive Capacity of this computer maybe on the small size

Meaning, the computer has a 256 GB Capacity Drive

A drive of this size, 256 GB in 2025 is akin to when Apple sold the Entry Level computer with 128 GB SSD drive

Unfortunately, the users' needs for Storage may have grown since the time it was originally purchased 

The implications being that on your next purchase.

Spend the Extra Money Up Front on a larger Drive Capacity Drive and add more RAM

This may just Future Proof the computer for your growing needs 

"Reducing 'System Data' storage on MacBook Air: [...]I found that 'System Data' is sth around 100GB. I remember it was like 50 GB when I used it a few months ago. How to decrease the size of it? I tried to get in terminal but I'm too scared if I'd break sth. Can you help me guys?"

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Thank you for the screenshot.

Ridding of System Data on a Mac:

Unfortunately, you're not going to rid of it, 100%, unless you were to format your drive and then start it all anew. Backing up your Mac prior to performing this format is what to perform. However, if you restore from this backup, all would return. So, you need to rid of this the best you can. See what is provided by Google: Reducing System Data storage on MacBook Air

System Data (formerly known as 'other') is mainly your own data stored within app containers. Email attachments are one, often significant, example. See:

What is “Other” storage on a Mac, and how… - Apple Community