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Nothing Phone builds a blue bubble iMessage bridge while Google and Apple fight over RCS
(www.androidcentral.com)
This is a most excellent place for technology news and articles.
They use a Mac mini somewhere to route these messages. So you're logging into that Mac mini with your iCloud credentials. Sounds like a privacy/security nightmare and creepy as fuck.
More likely just a Linux VM
iMessage only runs on Apple products
...or a virtualized Apple product on a Linux machine. iMessage doesn't know the difference.