Generally through the preamp out. Each head is often split into two portions. A preamp (signal creation) and a power amp (the bit that amplifies the signal and powers the speaker cabinet that is connected via a speaker cable to the head). The preamp out of a head is what you can use to send the signal out. Alternatively you can use that same output to a simple power amp itself (ie with no preamp). Others sometimes microphone their speaker cabinet and use that signal via its own preamp instead but that’s getting too technical and not often used for bass. That last option is more for studio work and less for gigging.

Reminds me of the music video for Metallica - One (MTV version) https://youtu.be/WM8bTdBs-cw

https://youtu.be/WM8bTdBs-cw