Question $3$: What do a "visited network" and a "home network" mean in $4$G$/5$G cellular architecture? Why do you need a tunnelling between the two?

Question $4$: When a mobile device is handed over from one base station to another in a $4$G$/5$G network, which network element makes the decision to initiate that handover? Whic network element chooses the target base station to which the mobile device will be handed over? Illustrate your answer.