And if so, what happens to those builders once the data center is built?
Yes. At some point the demand will be so high that imported workers won't suffice and local population will need to be trained and hired.
> And if so, what happens to those builders once the data center is built?
They are going to be moved to a new place where the datacenters will need to be built next. Mobility if the workforce was often cited as one of the greatest strengths of US economy.
More and more data centers (and power sources) are going to be built at the same time so more and more workers will be needed. This is going to be THE job. I think there are going to be many similarities with the age when railroads were being developed. Hopefully with less worker deaths this time.