As the arrivals terminal deals only with arrivals, there is no "air-side" there. If you take the bus to arrivals, you will have to clear customs and immigration, and officially enter the country.

If you have an onward flight with another carrier, you will have to collect your luggage at the arrivals terminal and then arrange for transportation to the departure terminal for your onward flight, where you will check-in and go through security once more.

However, if the inbound carrier was able to check you in on the onward flight, issue a boarding pass for that flight and interline your luggage, then perhaps you may be able to avoid the step of passing through the arrivals terminal. However, I wouldn't count on this being possible.