Q: DllNotFound: FirebaseCppApp-6_14_0 on Android only? I am currently using Unity to create my game, and I am implementing the user account system now. The send/receive methods for username/password work perfectly fine in Editor, but when I build on Android, logcat gives me the error in the question title (DllNotFound).
Obviously the DLL is not missing, yes? Otherwise it wouldn't work on standalone desktop/Editor? If this is the case, I am assuming it isn't getting built into the Unity .apk for whatever reason. Does anyone have any reason why? I haven't found any useful resources online, as all the similar questions I have run across are regarding no functionality at all, not split functionality across platforms. 
What reasons would Unity recognize a DLL in Editor/standalone, but not on an Android build?
(The .dll in question is located at \Assets\Firebase\Plugins\x86_64, if that helps).

A: Your fix is probably to run force resolve in the Android resolver.

Which should give you the file you need as part of firebase-app-unity:

My reasoning:
The Unity SDK actually ships with roughly five copies of each native library for iOS, Android, MacOS, Windows, and Linux (plus variants for different architectures &c). The FirebaseCppApp libraries being C++ implementations of some Firebase features. If you look in the folder you listed, you'll see a bunch of library folders for different editor platforms:
╰─$ ls -al | fgrep -v .meta
total 558360
drwxr-xr-x@ 14 martinpatrick  primarygroup        448 May  8 10:06 .
drwxr-xr-x  25 martinpatrick  primarygroup        800 May  8 10:18 ..
-rwxrwxr-x@  1 martinpatrick  primarygroup      48236 Dec  4  2016 FirebaseCppAnalytics.bundle
-rwxrwxr-x@  1 martinpatrick  primarygroup     355840 Dec  4  2016 FirebaseCppAnalytics.dll
-rwxrwxr-x@  1 martinpatrick  primarygroup     226136 Dec  4  2016 FirebaseCppAnalytics.so
-rwxrwxr-x@  1 martinpatrick  primarygroup   23186972 Dec  4  2016 FirebaseCppApp-6_14_0.bundle
-rwxrwxr-x@  1 martinpatrick  primarygroup    7526400 Dec  4  2016 FirebaseCppApp-6_14_0.dll
-rwxrwxr-x@  1 martinpatrick  primarygroup  254502608 Dec  4  2016 FirebaseCppApp-6_14_0.so

bundle is for OSX, dll is for Windows, and so is for Linux.
Clearly this is missing the iOS and Android variants of the SDKs.
The native iOS and Android packages are being pulled in via External Dependency Manager for You (see this post, Play Services Resolver is the old name). When it runs, it looks for a file matching the pattern *Dependencies.xml and starts looking for native libraries to pull in.
Firebase includes a local Android maven repository at Assets/Firebase/m2repository which contains firebase-app-unity-6.14.0.srcaar with the Android C++ implementation. If you're not using gradle to resolve dependencies, it should copy the necessary components into Assets/Plugins/Android on build depending on your settings in the resolver:

Note that if you're using gradle, this package will be added to your mainTemplate.gradle. If there's no mention of it, something may have gone wrong.
--Patrick
