Design the vertical frame on purpose
A normal 16:9 overlay will chop your cam, cover chat, or hide the game when a phone crops it. If Dual Format is available on your channel, build a second canvas: face in the safe area, alerts that do not sit on the gameplay you care about, and a chat position you can still read.
OBS and tools like Aitum are the usual way people build the two layouts. Test a recording before you promise the mobile audience a clean show. Availability depends on Twitch’s rollout, so check the dashboard before you rebuild the whole scene collection.