Detroit stood as the first major U.S. city to instate Prohibition"\ -- "instate" should be "institute"
A tunnel of psychedelic lights connects Detroit Metropolitan Airport’s A and B/C Councourses to an original soundtrack. -- "Councourses" should be "Concourses"
Almost as quickly as the law was enacted, underground smuggling and speakeasies began to appear, with some experts estimating Detroit’s river crossing to Canada -- I think Word thinks wants a "that" before "Detroit's." I don't agree.
Below ground, an elaborate tunnel system connected the Fisher to New Center and General Motors buildings to ease employee commute. -- should be "commutes" or "commute times."
A tunnel system connects each of the hospitals making safe, seamless travel -- comma after "hospitals", add "for" after "making"
The steam loops ran on coal until converting into natural gas in the 1970s -- should be "converted"
The unique system of service tunnels set a precedent for surrounding shopping centers, although they were off-limits to the public and remained closed and unpassable after the mall's closure in 2015 -- I would have written "impassable" but "unpassable" is correct
What else?