These are my current theories on puzzles, based on my observations. Subject to updates and revisions.
Puzzles can take form in many different ways. Examples of puzzles include jigsaws, crosswords, Sudoku. Within the context of Scrap.tf puzzles, they take the form of password-based enigmas.
There are two sides of every puzzle:
Puzzles considered difficult on Scrap.tf have their Understanding side left to the viewer, with the Execution typically trivial.
An example in which the Execution is difficult while Understanding is known is Spacechem by Zachtronics. In it, you learn about fake chemistry, and in every level you must take the inputs on a grid and transport them to the outputs. You know the rules from the very beginning, but executing a correct solution is the challenge given to the player.
Other examples include chess problems, Stephen's Sausage Roll, and most other puzzle games that exist.
For shorthand, I refer to the distinction of the puzzles above through Types:
Thus difficult Scrap.tf puzzles are Type I, while most puzzle games are Type II.
There also exists Type III and Type IV puzzles:
Type III puzzles are very straightforward, while on the other end of the spectrum Type IV puzzles are those near-impossible enigmas that are essentially unsolved mathematical problems.
Puzzles that require passwords as the solution typically take form in the ways I've observed below:
Of course, any puzzle may contain multiple archetypes and elements from each category.