I’m confused by Alfa though, is it just the Alfa Romeo sponsoring the team or did they build the car?
It's a sponsorship deal. Sauber now has the same 2018-spec Ferrari engine that Haas runs as opposed to the previous year spec they used to have in prior seasons.
As for their poor performance, the car's aero-package is a very low-downforce one so while it is fast in the three DRS zones, it's cornering speeds are the lowest on the grid so overall the lap time is as bad as it is.