As of today in 2019, Apple has made life much easier for low budget or one-man project developers like me. You can just use the terminal command from one of the above posts to record videos from the wanted device simulator. And then use iMovie's New App Preview feature.
xcrun /Applications/Xcode.app/Contents/Developer/usr/bin/simctl io booted recordVideo pro3new.mov
iMovie -> File -> New App Preview