Microtransactions in Temtem are summarized as...
The game was originally funded through Kickstarter. Since then the game has fully released and includes multiple different microtransactions as well as an initial purchase price. All microtransactions are cosmetic only.
The game features a premium currency called "Novas". These are used to purchase cosmetics and the Battle Pass.
The game features a Battle Pass that follows the usual system of expiring once the time is over. If you did not complete the Battle Pass you lose out on the cosmetics you didn't get, even if you paid for it with no other way of obtaining them.
This premium currency can be earned in-game through the Battle Pass without the need to purchase the Premium Battle Pass. This is done through leveling the Battle Pass as well as through Weekly Objectives in the Battle Pass. If you have the Premium Battle Pass you can earn more premium currency through more weekly objectives.
This means it's technically possible to obtain Premium Currencies and eventually purchase a Battle Pass without needing to spend money, but in reality this will take several seasons, but from there on out completing the Battle Pass will earn you enough currency to purchase each subsequent Battle Pass.