You'll reach this screen from two places — building or editing a routine, and tapping ＋ Add exercise mid-workout — and it works the same way either time.
Search by name, or narrow the list with the muscle group, equipment, type, and level filters; the result count updates as you go, and Clear filters resets everything at once. Tap a result to open its detail: setup, execution cues, common mistakes, and reference photos where the catalog has them.
From there, Customize lets you set the rep range, target RIR, and rest for this exercise before adding it, plus its starting sets and weights (or, for cardio, the mode, location, and target time/distance). When you're happy with it, Add to routine adds it and closes the picker, or Add & keep browsing adds it and returns you to the results so you can keep going. ‹ Back to results returns from a selected exercise's detail without adding it.
Added mid-workout, a new exercise prefills its starting weight from the most recent time you actually logged that lift, not a generic default.
Related: "Editing a routine exercise", "Starting a workout off-plan (Quick Workout)"