1. Run a command as every player
as changes *who* runs it, at changes *where* it runs — most of the time you need both together
/execute as @a at @s run say พร้อมแล้ว
What it does Will run the command "say พร้อมแล้ว" on behalf of all players at the position of themselves (whoever runs the command) (adjust the executor, position, and condition before the actual command runs).