You are asking legal questions: "What aspects of a program I have read
are covered by copyright? What kinds of similarities does copyright
apply to?"
The answers to questions like these are defined by copyright law, not
by the GPL. If what you do constitutes copying, as defined by
copyright law, then the law says you have to follow the distribution
terms of the program you copied--whether that is the GNU GPL or
anything else. If what you do is not considered copying, then the
distribution terms are irrelevant; legally, they don't apply to your
activity.
Just retyping something is legally the same as using a copy command.
Changing some of the details is making a derivative work, which is
legally still considered a kind of copying.
However, copyright does not cover general ideas and techniques. So
regardless of where you saw them, using them by writing your own code
is never a violation of copyright. (Unfortuately, some of these
general ideas and techniques are patented, but that is another story.)
Trivial little pieces of code that everyone uses are irrelevant for
copyright.
Those questions are legal ones, about what is lawful. What's right or
wrong is another matter.
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