What is the technology that transmits voice in 3G networks?
In 3G voice is transmitted digitally (i.e. as is data), however, the voice data is treated differently than other data.  In 4G there is only data, but a client application can utilize that data for a telephone call or other voice application using any protocol supported by both that client and some server with connectivity to the network.  Common VoIP standards include SIP and SCCP (skinny), but I'm sure there are more.

"when 3G coverage isn't good, change to use 2G networks for voice service" just means that you may well reach a 2G base station even when you can't reach a 3G one.  That could be for any number of reasons - range (if they aren't collocated), frequency band that each uses, and number of users using the service at that instant, for examples.