I have both R18A1 and R18Z1 engines.
i-VTEC ON = economy (lower lift) cam profile.
i-VTEC OFF = normal (higher lift) cam profile.
1500-3500 rpm is one of the conditions needed for the ECU to activate i-VTEC. Older VTEC was dependent on engine RPM only, while i-VTEC (with the i standing for intelligent) considers a bunch of other variables.
The engine has a variable length intake manifold. At and after 5200rpm, the shorter one is used. There is no "instead." The ECU decides when to "shorten" the manifold and when to engage the right cam profile.
Since this is 2024, and nobody knows how to read and write sensibly anymore, despite the great resource posted by @roony leaving absolutely no room for speculation, here's a YouTube video.