Based on our record, Facebook Design Resources seems to be more popular. It has been mentiond 2 times since March 2021. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Spotify isn't doing anything unique here, and it's not really a marketing campaign to explain away their UI. It's more of a showcase for hiring professionals, developers/designers/etc. Facebook has a similar one -> https://design.facebook.com/. Source: almost 3 years ago
Also, read anything published by Design @ Facebook. Source: about 3 years ago
Facebook Design - Devices - Images and Sketch files of popular devices.
Designer Mill - Collection of Best Free Design Resources
Device Shots - Generate high-resolution device mockups using a screenshot.
Interfacer - Collection of more than 200+ free design resources
Clay Mockups - Free set of beautiful device mockups for your designs
Freebiesbug - Collection of the best free web design resources.