You really don't want to read any of these. It's so satisfying to figure out the various puzzles and vague clues for yourself. Truly. Spend some time at it. Days, even. You'll be glad you did.
So why do I provide this page? Simply because there are quite a few repeatable missions that require some specific details that are fun and rewarding to figure out the hard way the first time, but after that it might seem tedious to figure out all over again some detail that you simply forgot. So this page is primarily meant for those types of quests and assumes you already figured out the mission one time already.
Also, there are some bugged quests where the clues you need aren't available in the mission journal when you actually need them, and if you forgot to write them down or screenshot them when you first saw them, you'll be totally stuck. So some of these are included here too for the folks who get messed up by a bug like this. For bugged quests like these, I provide only those hints/spoilers that you might need to get around the bugged parts of the quest.