A developer's Mac fills up differently from anyone else's. Photos and downloads are rarely the problem; build systems are. Every tool you use keeps its own cache, every project keeps its own dependencies, and almost none of them ever clean up after themselves.

The good news is that nearly all of it regenerates. The useful question is not "what can I delete" but "what does deleting it cost me" — a slower next build, a re-download, or something that does not come back.

Xcode and the simulators

Usually the largest single block of space on an Apple developer's Mac.

What it is Clearing it costs

DerivedData Build output and indexes, one folder per project A full rebuild and re-index — how to clear it

iOS Device Support Symbol files copied from each device you have plugged in A few minutes the next time that device connects

SwiftUI Previews Simulators created for Xcode's canvas Previews rebuild themselves

Simulator runtimes One per iOS or watchOS version you have downloaded Several gigabytes to download…