Jimmy is shown disassembling smoke detectors at Everett Acker's house. He is doing this to extract the small amount of the radioactive isotope americium-241 that they contain. This process of obtaining radioactive material from common consumer products in large quantities - in particular, smoke detectors - was famously employed by David Hahn, the "Nuclear Boy Scout", in the 1990s to build his homemade neutron source.
Steven Ogg reprises his role as a criminal named Sobchak from Season 1 Episode 9 Pimento and is now calling himself Mr. X. Originally Bill Burr was going to reprise his Breaking Bad role of Patrick Kuby, but when he was needed to shoot his scene, a friend of his passed away and as a result he was unable to return due to attending his friends funeral, so the part was rewritten for Sobchak instead of Kuby.