In such a case, the equipment and parts will be manufactured illegally in the USA and you can't shut down online distribution channels.
Then guns aren’t really banned are they in this scenario. I doubt there’s going to be a blanket ban anyway, but if there was then they’re not going to leave open internet shopping and courier delivery, or make parts readily available. That’s a series of restrictions with exceptions, not a ban.
And while it’s been a mildly amusing what if scenario, let’s look at what it would really take to implement a total ban on guns.
First the authorities would have to curtail voting
The control the election counts
Then work toward outlawing opposition and solidifying it’s power as a one party state
Then start implementing heavily enforced laws like gun bans
Then confiscating guns that are already out there
Then rounding up dissenters using deadly force when necessary
Well guess who’s started on the first two?
A ps it is now, a simple act of Congress might have a temporary ban on new sales of certain types with exceptions, amnesties, allowing those who already have them to keep them, allow people to make them with parts kits, stuff like the AWB. That doesn’t require a what if like the OP, it’s already happened or it’s already the case in some states.
But the idea that if some kind of dystopian govt implements a total ban? In that case they wouldn’t simply allow folks to buy the means online to make their own guns. If they enforced it, not only the finished product would be banned but parts, ammo, tools and materials and any cottage gunsmithing ‘industry’ would be driven deep underground.