You own a decent camera. You want it in a live stream. Everything you read says buy a capture card, and you would rather not.
Route one: the manufacturer webcam utility
Most camera makers now publish software that presents the camera to the computer as a webcam over USB. Free, and for a single camera it is often all you need.
The catches: it usually supports one camera at a time, the resolution is frequently capped at 1080p or below, and the camera may overheat or time out on long recordings.
Route two: a network encoder
A small HDMI encoder takes the camera output, joins your network and publishes an RTMP stream that software picks up as a source.
These cost less than most capture cards, work over Wi-Fi, and solve the separate problem of a camera too far away to cable. They add a little latency, which matters if you are cutting fast between cameras and does not if you are not.
Route three: the phone you already own
Worth saying plainly — a recent phone sensor outresolves a lot of what gets sold as a streaming camera. As a second or third angle, a phone on a tripod is not a compromise.
Lock exposure and focus before going live, shoot on the rear camera, and plug it in so the battery lasts.
When you should just buy the card
If you need more than two cameras, want them all at full quality, and want the lowest latency — buy a multi-channel capture card. It is the right tool, and it is also the purchase that lets you skip a hardware switcher entirely, which more than pays for it.
Mixing routes is normal
Most real setups end up mixed: one camera on a capture card, one on a webcam utility, two phones on Wi-Fi. Skynat Live takes all of these as ordinary inputs at once — up to twenty sources in any combination — and they can all be colour-matched so the cuts do not show which is which.