Through government funds, as other public works projects are. Or if people want to privately create their own, they can do so, as long as the results are open.
How are public works projects funded now? Same way. And if corporations want to fund certain projects too, they can. It's simple, extrapolate how we fund current projects and do the same with software, it's not that complicated.
Did you miss the parts where I mentioned that private individuals and corporations can also fund their own software, as we do today? My point is that as long as code, models, and data are open source, we will be free to mix and match and improve upon them, instead of only having rich corporations hold them. One way to do so is to have the government fund projects and open source them, as an alternative to corporate closed source control.