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Android will soon instantly log you in to your apps on new devices
(arstechnica.com)
This is a most excellent place for technology news and articles.
I think the smart switch app has special permission to read other apps' data, so it copies your session data to your new phone.
I just went through this the other week. I got a replacement phone identical to the one I had, and it worked for the most part. Most of the apps logged in but there's a good few that didn't and I had to do it manually.
It even copied over my browser cookies.
I was quite surprised by that too