Mac devices do not have a built-in method for creating a multi-output device, so you will need to download some additional software in order to achieve this. There are many options available, but this setup guide provides steps for how to use Soundflower.
You can find information about downloading and installing Soundflower here: https://soundflower.en.softonic.com/mac
Aggregate Devices can be used in place of a Multi-Output device when configuring I/O settings in a DAW that only allows one I/O Device. Aggregate Devices can also be used if one wishes to combine the I/Os of several interfaces.
To set up an Aggregate device you must access your Mac's Audio Midi Setup.
In order for OBS to use your DAW sound in your stream, you will need to set your Multi-Output Device as an output in your DAW. Steps for how to do that in some popular DAWs can be found below:
Garageband: Click GarageBand at the top > Preferences > Audio/Midi tab. Change Output Device to “Multi-Output Device”
FL Studio: Click File > Options > Audio settings > Audio tab > Under Input/output change device to “Multi-Output Device”
Ableton Live: Click Live > Preferences > Audio tab > Change Audio Output Device to “Multi-Output Device (0 in, 2 out)”
Pro Tools: Click Setup > Playback Engine > Select “Aggregate Device”