Two things on Instagram are called a broadcast, and people pick the wrong one all the time.
A broadcast channel is an Instagram feature. You create one, followers join it, and you post into it like a newsletter. They can react and vote in polls. They cannot reply.
A broadcast DM is one message sent into real conversations at once, through a DM tool on Instagram's API. It lands in the same thread as any other message, and people can and do reply.
The difference is not cosmetic. One builds attention you cannot act on; the other acts on attention you already have.
What each one actually reaches
A channel reaches everyone who joined it, whenever you post, for as long as they stay. There is no time limit and no qualifying action beyond joining. That is its real advantage and it is a large one.
A broadcast DM reaches everyone who contacted your account in the last 24 hours: a DM, a story reply, or a tap on a button in one of your automations. Not your followers, not your channel members, and not people who went quiet yesterday. Instagram does not let a business open a conversation, so a broadcast continues the ones already open.
So the channel wins on reach and loses on everything that happens next.
The thing most people miss
Nothing that happens inside a channel opens a messaging window.
Members can react to your post. Reactions are not messages, so they do not start anyone's 24 hours, and no automation can answer inside a channel. A channel with ten thousand members generates exactly zero reachable conversations on its own.
This catches people who assumed a channel was a warmer version of a follower list. It is warmer in attention and identical in messaging terms: you can talk at those people, and you cannot talk with them.
Which to use, by what you are trying to do
Announcing something to people who chose to hear from you is the channel's job. Members raised their hand, there is no clock, and nobody has to have messaged you this week.
Getting something into the hands of people who just engaged is the broadcast DM's job. A reel that did well this morning leaves a large group of open conversations, and a broadcast is how you reach all of them at once while they are still open.
Getting a reply only works in DMs. This is the one that decides most cases. If you want answers, questions, bookings or a conversation you can continue, it has to happen where people can write back.
The pairing that works
Use both, pointed at each other.
The channel is a megaphone with no capture: post into it and ask people to do something that generates a message. Comment a word on a specific post, reply to a story, tap a button. Each of those opens a window, and a window is a conversation you can act on and a contact record that is yours.
Then the DM side does the work the channel cannot: answering, collecting an email address that does not expire in a day, and broadcasting to everyone reachable when you have something to say.
Read that way the two stop competing. The channel earns attention; the DM turns attention into something you can act on and keep.