The way it's typically been handled is that the last mile ownership is still with the company that builds it, but they have to lease its use to other companies

The way it's typically been handled is that the last mile ownership is still with the company that builds it, but they have to lease its use to other companies at a reasonable rate. For places with no last mile infrastructure yet, one way to handle it that's been done is that the public builds it (i.e., it's owned by the municipal government) and leases its use also at a reasonable rate. An example in a different industry is electricity. I'm in Canada and in my province we pay to a local company, but a separate company owns the overall grid. There's a charge for distribution that appears on everyone's bill to pay for the infrastructure.