where X is a 10 digit phone number and Y is a 15 digit IMSI:
does the dial application assign the phone number X to the IMSI Y in this case? that is - is dial creating a channel between the phone number and the IMSI?
is the IP address in this line looking for the location of where the IMSI is stored in the db?
No. The Dial application requests the SIP channel technology driver to set up a connection of some sort, in this case a SIP session, passing it “IMSIYYYYYYYYYYYYYYY@192.168.x.x:5062” as a parameter. The channel driver returns a new channel. When the channel driver indicates that the call has been answered, the Dial application bridges the incoming channel to the new channel.
The SIP channel driver, sets up a SIP session with 192.168.x.x, passing it sip:IMSIYYYYYYYYYYYYYYY@192.168.x.x:5062 as the request URI. Asterisk has no idea what IMSIYYYYYYYYYYYYYYY means.
The peer presumably interprets the YYYYYYYYYYYYYYY as an international mobile subscriber ID, but we know noting about what is at 192.168.x.x:5062, so we can only guess this.
It seems a strange thing to do, as, as far as I know, the GSM network doesn’t allow calls to be addressed by subscriber ID, but that is not our problem, as it is the device at 192.168.x.x:5062 that deals with that problem. Normally you address them by the phone number corresponding to the HLR entry.