A dotfile is an ordinary file whose name starts with a dot. It often holds shell settings, but it has no effect until a shell reads it. source runs its commands in the current shell. Starting bash with the file instead creates a child environment, so its variable changes don't update the parent.
Inspect an unfamiliar configuration before sourcing it. An explicit export makes the difference between loading a setting and running it in a child shell visible. Use ls -a to find the file and cat to inspect its commands. source ./.routes and . ./.routes both read that file into the current session. The first dot in the second form is a command; the dot in the filename is part of its name. After loading it, print the exported value in the same shell.