How to add an CA to the default TrustStore in OSX and iOS

Both iOS and OS X can use configuration profiles to install certificates.

Apple has some nice documentation at https://help.apple.com/serverapp/mac/5.0/#/ for how to do that using Server.app. Luckily, you can use the same profile on iOS/OS X for certs although many other settings are unique to one or the other OS.

I would recommend using Apple Configurator 2 to make your profiles since it's free and graphical based. You might use Profile Manager on Server.app if you have lots of clients to configure and another MDM solution doesn't work for you.

No matter which tool you choose to craft the profile, it's just a file you place on a web server or email to the clients and then tap/click to install the certificate.