Maybe watching this will give you an idea of what went on during the stone age of audio recording, when there were no bits or bytes, "Plug-ins" were power cords 1/4" jacks, XLR or RCA connectors, and rercording consoles only had a couple of big knobs on them! They used tubes because transistors weren't invented, and people rehearsed until they knew the songs before hitting the big red button.