Award trips are never free. All have real taxes and with some ffp's/itineraries/routes have savage cash surcharges. Upgrades are but a distance hope. Not a realistic expectation in this part of the (frequent flyer) world.

Many ffp's require you to have 4 (or 3-2) flights on the ffp's own airline to get or maintain status. Other(very few?) ffp's do not have this requirement for status. Rules vary between ffp's.

With most ffp's an eligible activity with the ffp airline, partner airline or some mile earning activity will keep a ffp account alive. But ffp's do not all have the same rules. With some ffp's miles expire after a fixed X years:-use them or lose them.

As above with Air NZ many economy flights will be nil or very low earning.

USA based airline ffp's tend to be more generous (status benefits/earn/burn/upgrades/award cost/award cash surcharges/expiry/affiliated credit cards/promotions) compared to non USA based airline ffp’s (even after the recently announced changes to some USA ffp’s)

The airline you fly most or an airline of the country you live is the best ffp for many people. Generally however you are better earning on your primary airline due to greater recognition, better service when disruptions, ability to upgrade flights and possible operation upgrades.

The airline you fly and the airline ffp you credit those flights to does not need to be the same. Frequent flyer miles are not equal to earn or burn.