Boarding enemy ships is an essential part of Railjack battles in Warframe. While your Railjack can take out fighters, larger vessels like Crewships requires someone to go onboard and take it out from the inside.
To do so, you need to use your Archwing, and this is extremely dangerous. Run to one of the exit consoles, and interact with it to leave the Railjack. If you have leveled up enough, you should be able to use the Archwing Canon. Once outside, find the Crewship and head towards it in a long arc. You don’t want to head straight to it, as a direct hit from the cannons will kill you.
Move around behind it, get close, aiming for the door at the bar. Hit “X” to enter, and you will be onboard. You know have two options: take out the reactor or take out the crew.
Taking out the reactor causes the ship to explode, giving you a short time to get off. To leave the ship, run back to the door you started at and exit from there. You will enter Archwing mode, so be careful on the way back to your vessel.
If you take out the crew, you can go to the cabin, take out the pilot, and turn the Crewship on your enemies. Make sure to communicate with your squad because they can damage and even destroy the Crewship while you are on it. When you finish, go to the reactor and then leave it.
Head back to your vessel and aim for the doors on the sides, this allows you to return to the Railjack.