If you can read an accurate reading you can inject the correct fuel right?

wow.. thanks. I have learned something new today.

Shhssssh

They can run either one or both at the same time. Readings are measured all the time regardless of open or close loop from the o2 sensor. Btw, open loop means it's disregarding reading from o2 sensor. Particularly during cold starts or high rpm. Modern cars still uses MAF sensors. They provide accurate volume of air by measuring velocity of air inbound. However if there's leak at the intake interval it causes a huge problem as the measurement is off by a large scale. Map sensors however do not suffer such defects as it's installed at the intake manifold. Any leaks can be detected and ecu can compensate the changes. Downside is the accuracy is not as good as MAF. It relies on atmospheric pressure to estimate the amount of air being suck in. For engines with both system, they are either turbo charged or build for refinement and accuracy. N.A. wise, both MAF and MAP works in tandem to provide accurate volume of air being breath in.