The stolen boat allowed Artyom and Knyaz to enter the base of cultists. The following section of Metro Exodus guide discusses the last mission in the Volga location.
Regardless of the approach you assume, you reach the boat and the chapel of cultists. Again, an important choice has to be made - decide if you want a stealthy approach or an offensive one. Just like before, the choice affects the end and fate of your companion.
The offensive approach
There are a lot of enemies and in fact, even if you decide on a purely offensive variant, it is worth keeping even a little bit of discretion and eliminate the enemies quietly. Try to go up all the time and neutralize more enemies. Eventually, you will reach the lift - use it to move up.
At the top, you will see Silantius speaking to the congregation. As you don't care about discretion, open fire and kill everyone. Then run up the stairs and break through the door behind which the leader of a religious group is hiding.
Finishing Volga and deciding about Knyaz
You will now see a scene in which, among other things, the fate of Knyaz will be determined. If you went through this level quietly and stunned your enemies instead of killing them, the man will survive and make it until the end of the journey. If, on the other hand, you murdered others, the man will be wounded and die.