You have about half a dozen guys on the Roster eating up the majority of cap space. You fill in the other positions with late round or undrafted players, practice squad sign ups, etc. Those 'non-core pieces' add kind of mix and match. Most them are, as you would expect, inconsistent. They can have a great game and then string a few stinkers together.

Elite players (or core players) play at a high level week in and week out - these are usually higher round picks or vets making money.

Sometimes you get breakthroughs from the lower ranks who grow to become elite. Not many of them sustain that level of play, though it seems. See a lot of the FA's the Rams have let walk and track how they're doing 2,3,4 years down the road.