I am sure there are many options to do this; here are some I have used.
camtwist. (macOS) Camtwist applies live video effects oddities to your webcam and creates virtual cameras out of screen captures. I have used this one. It is simple and handy, like OBS but does not explode so often.
OBS is a maximalist open-source screen broadcasting package that does everything including running VST effects on your audio stream. Oriented to live use. Somewhat quirky. Windows/Mac/Linux. Linux support is reportedly restricted by choppy support in Wayland, the modern windowing system.
Syphon is an open source Mac OS X technology that allows applications to share frames - full frame rate video or stills - with one another in realtime. Now you can leverage the expressive power of a plethora of tools to mix, mash, edit, sample, texture-map, synthesize, and present your imagery using the best tool for each part of the job. Syphon gives you flexibility to break out of single-app solutions and mix creative applications to suit your needs.
Spout leverages your graphics card enabling to you send realtime video between Windows applications with near-zero latency or overhead. It is provided free and open source thanks to support from its users
Disclaimer: I know the author of spout, and he is bloody lovely.