How to launch lldb as a Standalone Debugger on an app running on a simulator?

My question is a follow-up to How can I use Swift REPL with iOS SDK: I'd like to be able to drop into a REPL in an iOS project and use the classes defined there.

I've already built the project and it's running on a simulator:

Now, I'd like to start a standalone LLDB debugger for the iOS project, following these commands quoted from the answer above:

$ xcrun lldb -- $DerivedData/$AppName/Build/Products/Debug-iphonesimulator/$AppName.app
(lldb) process attach --name '$AppName' --waitfor

However, it is unclear to me what to substitute for $DerivedData and $AppName. The LLDB Quick Start guide, https://developer.apple.com/library/archive/documentation/IDEs/Conceptual/gdb_to_lldb_transition_guide/document/lldb-terminal-workflow-tutorial.html, didn't help me much either, as the example just provides a path without explaining how you get that path.

I've also tried following the answers at https://apple.stackexchange.com/questions/171752/ios-simulator-installed-app-location-in-xcode-6-1, I've tried searching in the location below, but didn't find it there:

$ find . -name '*app'
./Developer/Library/Xcode/Agents/XCTRunner.app
$ pwd
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform

How do I find the .app that is running on the simulator to launch lldb with?

Update

As pointed out by Josh Caswell, DerivedData is located in my home directory, and there I can find the app which is supposedly running on the simulator:

$ cd ~/Library/Developer/Xcode/DerivedData
$ find . -name '*Venmo.app'
./VenmoWorkspace-apbzqcexodswkdfenhxlkpnlmvsx/Build/Products/Debug-iphonesimulator/Venmo.app

The only problem is that if I try to run the repl command, I get error: Can't run the REPL without a live process.:

$ lldb ./VenmoWorkspace-apbzqcexodswkdfenhxlkpnlmvsx/Build/Products/Debug-iphonesimulator/Venmo.app
(lldb) target create "./VenmoWorkspace-apbzqcexodswkdfenhxlkpnlmvsx/Build/Products/Debug-iphonesimulator/Venmo.app"
Current executable set to './VenmoWorkspace-apbzqcexodswkdfenhxlkpnlmvsx/Build/Products/Debug-iphonesimulator/Venmo.app' (x86_64).
(lldb) repl
error: Can't run the REPL without a live process.

Why is the app running in the simulator not being 'picked up' as a live process?

Update 2

Reading the instructions more carefully, it seems I have to attach to the process first:

(lldb) process attach --name 'Venmo' --waitfor

The problem now is that this command is 'hanging'. Any idea why lldb is not able to attach to the app's process?

$DerivedData is just the path to your project's Derived Data directory. The default for this is ~/Library/Developer/Xcode/DerivedData You can see the current value in Xcode settings:

Your project will have its own directory in there, which is the first $AppName. $AppName in $AppName.app is the target's Product Name from the project settings, which should just be "the name of the app" unless you've been messing with those settings.

You can also get the values for your project using xcodebuild: xcodebuild -project myapp.xcodeproj -showBuildSettings