Depends on the cadence for each team...
Many use a variance of:
1. Quick snap: any word, such as "set", "go", "hut"
2. On a count: such as on 2 or 3. Some teams use a repeatable phrase, ex: green Bay uses 'green 18'..so on 2, would be green 18, green 18.(ball snapped)
3. On a specific word: such as 'set' or 'hut' or 'go'. ex: on 'set'..green 18 green 18 ready set (ball snapped)
4. Silent count: usually reserved for loud road games, not typical this yr
5. etc.
Bottom line each team does their own thing, and has certain ways to cadence, and also try to keep the defense off balance/less of a jump, or even draw Offside. Alot comes down to QB voice pitches, can they adjust "hard" counts without getting their own oline to false start.