The simple answer is that they would have known what they needed to know to survive. The best example is anyone navigating their hometown or an area they have lived in for awhile. You may not know every little detail of the area, but you know where the gas stations are or where cops like to set up speed traps. You know how to get to the dog park or how long it should take to get to the grocery store with traffic.

Imagine then if someone new came through and began asking questions about your community, except it was for things like "What is the average time spent in traffic on Lathrop when driving at 3pm on a Tuesday? Can you still get to the store on time?" Maybe you have some rough estimate, or maybe some can even be answered. But for the most part they are looking at things from a perspective you have not had to up until this point. You don't need to know if Highway 31 can support trucks over a certain weight class.

This is sort of the issue with looking at what the Corps of Discovery recorded verses what local cultures already knew. They may have known that there were mountains at the edge of the plains, but it's kinda like being asked where a landmark is in town from a specific location. You might be able to guess it's a certain number of blocks away or a 15 minute drive, but most of us would be at a loss when asked for specific distances and measurements to that location.

In the Upper Mississippi River Valley we know that communities would spend some months along the river, and then would spend another portion of the year on the plains hunting bison. They knew how far it was to get to the plains from the Mississippi and may have heard information about things like the Rockies or even the Gulf coast. A tribe living along the Missouri may have lived in a similar manner, except they have a better grasp on the scale of the Rockies but a lot less of an understanding on the Gulf coast. What they knew about the geography of the area depended on where they were and where they directly travelled to. Ultimately you don't need to worry about where the pacific is if you are living in Iowa, but you need to know all about Iowa to survive.

Without any central entity compiling these native accounts, what would have been known in each community would have depended on who they interacted with and what information was needed to survive. Lewis and Clark set out knowing what potentially were the boundaries of the area they were heading into. They had heard the accounts of French trappers in the Rockies or British sailors in the Pacific. They came into this region with a view of it that the local tribes had never needed nor had access to.

Tldr: the local communities individually did not have any means of fully documenting the western US, but would have known their own local regions quite well. Lewis and Clark roughly knew the big picture, but would have needed the expert local accounts to get a better understanding of what truly was out there.