Short answers, pulled from the story.
Microsoft unveiled Windows Phone 7 in February 2010 at Mobile World Congress in Barcelona, Spain. The platform was built entirely around the Metro design language, which abandoned traditional grid icons for flat, typography-driven Live Tiles.
Microsoft CEO Steve Ballmer and Nokia CEO Stephen Elop announced a strategic partnership in London on the 11th of February 2011. This alliance made Windows Phone the primary operating system for Nokia devices and replaced the aging Symbian platform.
No existing Windows Phone 7 device could be upgraded to Windows Phone 8 because Microsoft replaced the Windows CE-based kernel with the Windows NT kernel used by PC versions of Windows. This architectural shift created a catastrophic incompatibility that left older hardware behind despite supporting new apps.
High-profile apps like American Airlines, NBC, and Pinterest were discontinued by 2015, and Snapchat refused to develop an official app for Windows Phone users. Google also blocked Microsoft's own YouTube app twice for violating terms of service.
Active development of Windows 10 Mobile ceased in 2017, and the platform was declared end of life on the 14th of January 2020. Market share sank to 1% in 2016, leading Microsoft to prioritize software development and integrations with Android and iOS.