In each footage from ISS I see this thick tube going through the hatches into Soyuz spacecraft, ending in the descent module.
What is it for?
In microgravity, hot/cold air does not move the same way, so you do not get natural mixing or movement of air.
Sleeping astronauts get a air stream pointed at them so that they do not get enveloped in a cloud of carbon dioxide and die.
When docked at the ISS the Soyuz is mostly powered down, or runs of the ISS power and life support systems. After all they are designed to stay docked for 200= days.
Mir was full of these things between the modules. The ISS design was a bit better and seems to require fewer. But the Soyuz vehicles still need the ventilation.