General info:
There has to be a battery cable hooked to the large post, and on that large post is generally a feed to the interior.
When you turn the key, the feed from the interior send 12V to a trigger wire, that in turn engages the solenoid, which engages the starter. A solenoid (and starter) needs to be grounded for that to work.
Sometimes, starters have a trigger wire and a "cold start" ignition wire, if you had those reversed, or on the wrong lug, it would not engage
But, if you have 12V to the trigger (usually labeled S) when turning the key AND you have a good ground, if the starter doesn't do anything, it's a bad starter (or solenoid if mounted on the starter)
Sometimes tightening either the trigger wire or the batt cable feed itself can break off a connection internally, especially if the nut holding the stud was loose when you did it.
FYI, I like test lights better than volt meters in this instance.