The new NASA video Science Launching on SpaceX's 23rd Cargo Resupply Mission to the Space Station a bit after 01:20 shows a large experimental apparatus which looks a bit like a pressure vessel inside a protective frame being pulled out from what looks like another pressure vessel's hatch flush with the side of an experimental area aboard the ISS.
Question: What is this complicated experimental apparatus being pulled out of another complicated apparatus inside the ISS?
It is the Multi-user Droplet Combustion Apparatus, inside the station's Combustion Integrated Rack (CIR.) The CIR houses hardware capable of performing combustion experiments to further research of combustion in microgravity. https://spaceflight101.com/cygnus-oa5/cargo-manifest https://en.wikipedia.org/wiki/Combustion_Integrated_Rack
That is the Combustion Integrated Rack (CIR) with its MDCA (Multi-user Droplet Combustion Apparatus) insert. This paper has a lot of technical details on CIR.
The MDCA is capable of dispensing and igniting various liquid fuels with different mixtures of oxygen and inert gasses to study combustion in microgravity.
There's no convection due to atmospheric density, so the flames are spherical as shown below.
That portion of video you linked to was actually taken during Expedition 44/45 in 2015, since it shows astronaut Kjell Lindgren handling the MDCA. In between the time the original video clip was recorded and when NASA posted this video, the MDCA had been replaced by the ACME (Advanced Combustion via Microgravity Experiments) insert. The inserts were swapped on October 11, 2017.
Shown below is ESA astronaut Paolo Nespoli wit the CIR optics bench translated out and rotated for maintenance. This was still with the MDCA insert which was later replaced by the ACME insert on the same Expedition 52/53.
The ACME insert (shown below) allows for tests of gaseous fuels. It was one of my favorite experiments to watch during a slow shift on console. Here's a video of a particularly interesting test point.
