What is the difference, if any, between different Gapps providers such as opengapps, tk gapps, pa gapps etc?
Open gapps is open source developed, all their code is available on github. PAGapps is developed by the Paranoid Android team. So there you have it. An app doesnt have to be created by google to work as a google app. Other developers also give you a choice as to whether you want all or just one google app (gapp) The pico is just the play store, if I remember rightly. I no longer use any Gapps at all. I use a separate apk downloader called Raccoon with a throwaway google account.