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Inquisitors theoretically have the power to command the entire Imperium as they claim only to serve (and by extension are accountable to) the Emperor Himself.
In practice, it varies and it's almost down to celebrity status. An Inquisitor such as Coteaz or Karamazov can command Guard regiments, Sororitas convents, and Navy squadrons. Space Marine chapters are usually requisitioned via honour pacts or debts incurred by said Inquisitors.
A low ranking Inquisitor will usually command a group of handpicked individuals, see the Inquisitor game for an idea of how that works.
An interaction would probably go something like this:
Inquisitor "Governor give me all your forces in the sector."
Sector Governor "Haha no, get out of here you rat."
Same Inquisitor "Captain I require your platoon."
Guard Captain "Yes sir, right away."

The utter randomness is something that I hate about the Inquisition and I much prefer when they're portrayed as powerful but with more defined limits to that power. People can say all they want that there's no chain of command in the Imperium but there absolutely is.
Emperor > High Lords > Sector Gov > System Gov > Planetary Gov > Guard, Navy, Arbites. Astartes almost always operate outside of the chain of command and fight where they deem it necessary but will usually not command Imperial crusades. Of course, exceptions exist such as with the Astartes Praesus who guard the Eye of Terror and the Adeptus Vaelarii who guard Elara's Veil almost exclusively operate within these regions.
Sororitas tend to guard holy worlds or go on holy crusades and fall under the command of whoever leads said crusade.
With the Guard, ranks are still ranks and a Cadian captain would not command a Catachan colonel. They would only likely take command of PDF forces.
An Inquisitor often usually works the same way as they have their own ranks within the ordos. An Ordo Malleus Inquisitor would likely command a Grey Knight strike force but not a Sororitas Priory or Deathwatch Kill Team.
Inquisitors are, by and large, additions to an army rather than commanders