How does the federal government's financial support of medicaid under the ACA work

Factual Questions
1

I’m under the impression that under ACA regarding the expansion of medicaid, the federal government pays 90-100% of the cost of the program.

But is that indefinite, or is that subsidy going to decline over time and the costs be passed onto the states?

2

According to this site, 2020 and beyond the government will pay 90% of the cost. As of now, there are still 19 states that have not expanded Medicaid.

3

The federal government continues to pay a portion of Medicaid costs for those persons who were eligible for Medicaid per-PPACA. The federal contribution varies by state according to a rather incomprehensible formula. For those persons the federal contribution is roughly 75-80% of the total premium. The states pick up the rest.

With the PPACA the federal government demanded that states expand the pool of eligible persons for Medicaid coverage with the federal government picking up 100% of the premium only for those newly eligible and only until 2016.

Kaiser Family Foundation notes:

Of course the Medicaid expansion requirement was ruled unconstitutional by the Supreme Court and many states chose not to expand Medicaid. In those states the federal government continues to provide a federal contribution at previous rates to those previously eligible for Medicaid coverage.