Macs are around the five year mark for a full ARM transition. Remaining Intel Macs are right on the edge of not receiving software updates anymore, and the Rosetta translation layer already has a scheduled wind down.

Any Mac application will be built for ARM at this point, and anything made for Intel Macs will run seamlessly under Rosetta. And that stuff is mostly limited to developers making Intel Docker images, musicians using some VSTs that haven't upgraded, and games.

This is at least the third major architecture migration for Macs, and they always rip the band aid off and applications have to upgrade or not run. (Motorola to PowerPC, PowerPC to Intel, Intel to ARM.)

Approximately 100%. All MacBooks have been ARM based for years now.