How does unity remove useless things from unity projects??

Select the scene you want to use, right-click Select Dependencies, which will select all the files used in this scenario and export the files as. Unitypackage.

In the same way, export the package separately for all the files used in several scenarios. Delete all other files except the scene file in the project and import the previously exported package.

In this way, the files in the project are the files used in the scene.

Before these operations, the data should be backed up to prevent mis-operation.

Or go online and look for a script that detects resources.

Reference: http://www.ceeger.com/forum/read.php?tid=10220

How does unity remove useless things from unity projects??