The standardized ecu was introduced as a cost savings and fairness measure in 2008. The contract was won by McLaren Applied (which was part of the same McLaren that is the current reigning champion but is now spun off and owned by private equity).
The teams basically get an sdk and are largely responsible for the software the cars run at pretty much everything above the hardware layer of abstraction.
As a weird piece of history, McLaren applied was not the original winner of the contract for the first standardized ecu in 2008. When it was announced, in 2006, Microsoft was.