Apple Watch doesn't export FIT files natively, but third-party apps bridge the gap.
Apple Watch saves workouts to Apple Health, not as FIT files. Use a third-party app to export: HealthFit (simplest), RunGap, or WorkOutDoors (records with FIT export built in).
HealthFit reads your Apple Health data and lets you export any workout as a .fit file. That file works in MetricsOverlay like any other.
Available data depends on workout type. Heart rate is always recorded. GPS for outdoor activities. Speed, pace, distance from GPS or accelerometer.
Apple Watch Ultra has dual-frequency GPS which produces cleaner tracks for the map overlay.