Well, firstly, most of them on that kind of schedule, they're making their full-time wage making comics. I have a part-time job at an edutainment company three days a week and then make two pages of my webcomic a week as well, but if my webcomic was paying my rent, I might be able to make four pages a week.
Then, their comics are usually making them enough money that they can use some of that to pay assistants to do simple, repetitive tasks like flatting colours, inking, lettering and speech bubbles etc or even doing the social media stuff. So that can save time.
Then a lot of them use shortcuts. Things like 3D models for backgrounds, drawing a head turnaround for characters that they can copy-paste into panels, having colour palettes set up for each character to sample and drop in. They usually work in a simple style; minimal shading, simple lines, often unedited 3D models dropped into backgrounds etc.
Finally... and I don't recommend this one... some of them...er...just don't have lives. They don't eat well, they don't work out or go out, they don't have a partner or spend time with family or friends... They just... make comics all day every day. It's not particularly healthy and usually causes burnout or injuries.