Just as the beginning starts where things change, the ending should end where things return to normal, or near enough.
In stand-alone novels, all loose strings are tied in a nice bow. The ending reminds us of what happened at the beginning. Things that were important there need to be important at the end too.
Unresolved endings are usually for series, wherein the main plot appears to be mostly answered, but there are enough loose ends and questions that will lead us into the next book. Mystery series, detective series and police procedurals are those. The mystery is solved, but the character’s life continues and advances over the series.
Thriller and horror series use unresolved endings by leaving the solution of the main plot somewhat open. What good is a guy coming back from the dead if you don’t see the hand coming out of the grave at the end of the previous book? Or the monster’s child hiding in the shadows?
However you decide to end your book, you need to give a falling action to every plot and subplot. Things are resolved, at least to a livable point. Life returns to close to normal.
The most important thing to remember is that a reader remembers the last thing read. Always. Make your ending memorable. Make your reader want to buy another of your books.

