The short answer
A Live Activity is designed for timely state, not a permanently running second-by-second process. An app can update its activity while it has execution time, including some background opportunities, and a server can update it with ActivityKit push notifications.
Push updates have a budget
Apple documents a system budget for ActivityKit push notifications. High-priority updates count toward that budget and may be throttled if an app exceeds it. Apps that genuinely need more frequent push updates can declare frequent-update support, but the person can still turn that capability off.
What that means for step counts
A good step Live Activity should communicate freshness honestly and stay useful even when the number is not changing every second. StepIsland treats the surface as glanceable progress, not as a promise of a continuously executing pedometer.
Primary sources: Apple — ActivityKit push updates · Apple — updating a Live Activity