What is SDKExplicitPrecompiledModules in DerivedData?
Quick answer
~/Library/Developer/Xcode/DerivedData/SDKExplicitPrecompiledModules holds precompiled modules from Apple's SDKs (.pcm files) that Xcode reuses across projects when it builds modules explicitly. It's a cache: safe to delete with Xcode closed, and rebuilt on the next build.
What it is
On our test Mac it was 182 MB of .pcm files named after SDK modules, like _AvailabilityInternal-….pcm and _Builtin_float-….pcm. They're shared by every project, which is why the folder sits next to the per-project folders instead of inside one.
Can I delete it?
Yes, with Xcode closed:
rm -rf ~/Library/Developer/Xcode/DerivedData/SDKExplicitPrecompiledModulesThe next build of each project takes a little longer while the modules are compiled again.
Questions
Is it the same as ModuleCache.noindex?
Both hold compiled modules shared by all projects; this one holds precompiled SDK modules. Both are caches and both are safe to delete.
Will it come back?
Yes, on the next build that needs those modules.