I read it recently, maybe 2021 or later. It had been written recently.
The astronaut landed on the structure in our solar system. He landed with a team but for most of the novel he is exploring alone.
The struture is vast and contains a huge network of caves and tunnels. Near where he first enters it, there is a partly constructed red alien spaceship. No aliens are in it. Inside the network of caves and tunnels he encounters very many different alien species.Some sentient, some not. Often he is unsure. Most seem to have been living there for generations.
At one point he teams up with a group of obviously intelligent creatures much smaller than him. He cannot understand their language. He protects them when they are attacked. He joins in a ceremony when they mourn one of their number who has died. They start to enter a place he cannot follow. When they realise he cannot come with them there is a sad farewell ritual.
He eats some of the alien animals. He has to eat and at first hopes he isn't eating any intelligent people. Later he eats obviously intelligent dwarves that attacked him and had language. He thinks he might as well eat them now they are dead. The meat is much easier for him to digest than most aliens, close to his own biochemistry. Amongst the dwarves' posessions he finds supplies from his expedition with labels in English. He wonders if they killed humans for them or traded for them.
The environment seems to be artificial and every so often there will be changes in the size and the direction of the gravitational field and the composition of the atmosphere. Sometimes be temporarily enters an environment in which he cannot breathe and has to quickly go back through to where he can live. Many of the environments are unpleasant for him but not deadly.
The tunnels sometimes exit onto obviously alien planets in far solar systems. He explores one for a while. He is lost and does not think he will ever get home.
The dwarves who attacked him, because he had been killing members of their species, were humans from his expedition! Some transformation made him grow to twice his size and hear human speech as incomprehensible squeaking.
