Advent of Code is an Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like. People use them as a speed contest, interview prep, company training, university coursework, practice problems, or to challenge each other.
You don't need a computer science background to participate - just a little programming knowledge and some problem solving skills will get you pretty far. Nor do you need a fancy computer; every problem has a solution that completes in at most 15 seconds on ten-year-old hardware.
I (mostly) try to write both efficient and maintainable code. There are most likely fancier and more efficient solutions out there. I also try to cover any edge cases within the task description (or make my code re-usable [one example is day 17 which was reused with some adaptions on day 24]) even if that code will never be executed (e.g. because I checked my input and it is simply impossible for that to run in any case).
All solutions were thought off and implemented by myself. There was no code-copying or cheating going on (except for the chinese remainder... I only missed one math lecture during my bachelors degree and I'm pretty sure that's why my colleagues were so fast that day).
I probably used the boost library way too much to define grammars and parse input, but I like the security and the fact that I won't need to do any "lookahead" code myself when parsing the input.
I compile my solutions using g++ on Debian (you can find details about my Debian configuration here)
$ g++ --version
g++ (Debian 10.2.1-1) 10.2.1 20201207
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE
My solutions all try to read the input from a file called input.txt in the execution directory. This is true even for the days, when the input was given directly in the task description. I usually run g++ ./main.cpp && time ./a.out to compile and stop the execution time.