That's not important, because it's just a nonsensical plot device that the author put no thought into. It wasn't even the main Bulma that viewers are familiar with who invented the time machine, but an alternate version of her that never even appears in the story proper. We never learn how she made it or even see her do it as it's completely swept off-panel. If it was truly something important in the story, then it would have been fleshed out in detail for us viewers to see because only then would it be an actual point of focus. But it wasn't, because no one gives a shit about it. You could have had Trunks travel back in time through a wormhole inside of his toilet and nothing would actually change.
Dragon Ball isn't about scientists building shit; it's just about fighting at the end of the day, which is the only thing that people tune in to see. People don't tune in for Bulma; they tune in because the wanna see Goku, Gohan, Vegeta, Trunks, Piccolo, etc fuck someone up. There isn't much to the plot at all, because it's full of holes and none of it actually makes any sense. Dumb shit like time machines and time traveling only amount to arbitrary plot devices to set the stage for the fighting; it was never necessary for Toriyama to write any of that stupid bullshit when he easily could have written something less stupid. Even he admits that the Android/Cell arc was an incoherent mess.
You don't need time machines, you don't need multiple timelines, you don't even need androids. As long as Goku's turning his hair golden and having intense battles with someone, it doesn't matter what the so-called "plot" is because that's all the series needs to make money. So, yeah, Bulma's useless; she doesn't need to be in the story at all.