Copyright doesn't cover the results of code, nor the methods used in the code, techniques and algorithms aren't covered by copyright. Period. Copyright applies to 'the work'. If you don't copy the source code, it's not covered.
The elements of a design that are ornamental, utilitarian, or a general look and feel are not covered under copyright but would be covered under a design patent if one exists.
Strings are only covered under copyright if they are a sufficiently original work of human expression. Simple informational messages generally wouldn't qualify.
If I take the first chapter of Harry Potter and write a new ending, it's still copyright violation. They took many parts.