Welcome to Flyer Talk SkipDrake

I assume you are referring to paid standard *A RTWs.

In this case each flight counts as long as it requires a separate "coupon". In general this means every time you change flight number, or stop for a day or more, there is a new coupon.

Eg NZ2 AKL-LHR may be one coupon even though transits LAX, but if you choose to stop one night in LAX and continue on NZ2 the next day that would require 2 coupons. NZ6 AKL-LAX connecting to NZ2 LAX-LHR would require 2 coupons regardless of whether you are continuing same day (just a few hours transit at LAX) or stopping over.

Each coupon has mileage from start to end point. This will be close to the amount on great circle mapper, but not identical. The amounts determined may vary slightly depending on which airline issues the ticket.

Mileage earned depends on the rules of your frequent flyer program. The base amount would be from that airline's calculation of the distances, but with minimum mileage, status bonus, class of service bonus, etc factored in according to that program's rules (often varying by the airline operating the flight too).