For names to autocomplete in filea.c without being recognised as symbols you must have `Edit->Preferences->Editor->Autocomplete all words in document` set.
As the name says it autocompletes words _in the document_, not between documents.
This setting is for use in documentation when there is no symbol detection. For programming languages, symbol definitions are detected to provide additional useful information such as function parameters and to limit autocomplete to the same language, but you need to see the definitions in an open file or loaded tag file.
When any word in a document is considered for autocomplete, making all words, in all documents open, available in all other open documents will likely result in too many autocompletes to be useful, and probably won't happen.