It's been 40,000 kilometers with no major issues. What maintenance should be done?

My car has done 40,000 kilometers, and I feel like there's an issue with the transmission. Have any of you changed the transmission oil or had it checked? Looking for advice.

What maintenance should be done at 37,000 kilometers?

Asking everyone, with the car being three years old and having 20,000 kilometers, what maintenance items need to be done?

What maintenance should be done at 100,000 kilometers? Do the transmission oil and gear oil need to be replaced?

What maintenance is typically needed for a 60,000-kilometer major service?

For a car from 2013 with 190,000 kilometers, and considering the mechanic mentioned there are issues, what kind of maintenance should be done?

It's been 40,000 kilometers with no major issues. What maintenance should be done?

What maintenance should be done when the car is about to reach 60,000 kilometers?

How much does a major maintenance for a Subaru Forester cost at 60,000 kilometers?

What maintenance items should be done for a 2021 Santana with 65,000 kilometers?