I know that not all planets rotate at the same rate as our lovely planet. I need some help developing things on a planet I made for a personal worldbuilding project of mine.

Picture this: this is a planet beyond our solar system, and it definitely doesn't rotate at the same rate as Earth. In fact, days and nights each last for a decade. If this planet has decade-long days and nights, what would the climates and lifeforms of this planet be like? Would creatures on this planet sleep for a decade at a time during the cycles? Would it experience seasons? What would the climate be like?

There are no wrong ideas here.

(For reference, there are trees on Earth that can actually go for decades without sunlight)

7898765
My best guess is that the flora and fauna of this world have adapted to survive both the mostly cold nights and hot days by the use of an anti-freezing and -melting/burning gene in their DNA.

7898775
That makes sense. Maybe most creatures would be nocturnal or crepuscular.

Your thinking seems ok, but wordwise you seem to have confused 'orbiting' with 'rotating' https://www.generationgenius.com/earth-rotation-and-orbit/

7898924
Thanks. Then my planet is slow-rotating. You have any thoughts on what I'm asking about it, though?

I think if you have weather, life, and inhabitants on a planet with days and nights a decade long each, the sunny side would get very hot, and night side would get very cold, and the weather would be pretty extreme,

Instead of sessile organisms adapted to surviving alternating decades of blasting heat and crushing ice, I would imaging mobile organisms following the window of Goldilocks conditions as the planet rotates. Twenty years to migrate 40,000 km works out to 40,000 km/7,305 Earth days = 5 and a half kilometers a day or 0.2 km per hour to keep up with the optimal conditions. Comfortable walking speed for humans is about 5 km/hour, horses somewhere around 7 km/hour (see wikipedia), a millipede travels at roughly 70m/hour (Youtube v=LrxZfozFeHc) and a healthy paramecium swims at around 6.7 m/hour. So it would be harsh on protozoans and millipedes (cockroaches can boot it at around 5 km/hour) but land vertebrates could keep up. Once life gained a foothold (and a foot) on your planet, there would be a strong evolutionary pressure favoring fast movers. Cockroaches. Planet roach.