Question about Beyerdynamic DT 900 PRO X

No, you don't necessarily need an amplifier, the headphones are already quite good in the tram. But as soon as you want to listen to music with them, you need an amplifier, of course. Every headphone needs one, some already have one built in, but not this one. Either the source device needs an amplifier or one is connected between the source device and the headphones.
Wherever there is a built-in headphone jack, there is already a headphone amplifier in it.

No, you don't necessarily have to.