OK, think I got it:
-) there is still a (butterfly?) valve regulating the ingress of air into the manifold, under control of the "power" lever
-) in the manifold, fuel is injected, mostly close to each individual inlet valve
-) fuel injection is metered to produce a given air/petrol mixture, as determined by manifold pressure, temperature, and probably the setting of a "mixture" lever.
Modern engines have electronic ECU's to control all this, though I understand early injection engines (RR Merlin?) did this mechanically.
Thanks for the replies!