I get the general idea. But, how do they work from start to finish; how can you tell them from real people posts; what do you look for to find a bot?
What percentage of posters on social media are paid to post? Do we know?
And why do social media companies not try to shut down bots?
SlashElectric gave you a great explanation. From a political point of view (my background), I can tell you that they are invaluable nowadays. They are being used more and more to help shape, or reinforce public opinion. Go to any Fox News, CNN, CBS (etc) post on social media, and you will see the comments riddled with "bot accounts", all transmitting a variety of interconnected messages. Often times, the "top comments" will be from bot accounts, and then the subsequent comments will be a mixture of bot and real accounts. This helps shape a certain narrative that certain actors want you to believe; hammering home certain key words and phrases.
Twitter, especially, has a major issue on their hands as bot accounts are used frequently to get misinformation trending. The information will start with bot accounts, and then will be shared by people who may agree (or disagree) with the message. You see this a lot with the conspiracy theory crowd (such as the QANON groups). This is why people have to be careful with what they share.
I don't think we know the percentages, but it's quite a bit. The easiest way to spot a bot account is when they continually use talking points, or post the same comments over and over. When you select their profile, most likely they will have a generic picture, not a lot of history, or have huge periods where they are active (and then inactive). Also, especially on Facebook, if all that account does is post political material, they most likely are a bot account (i.e. nothing personal). Twitter it is harder to manage because most people don't post very personal things on that platform.
Social media companies are trying to combat this, but there isn't a lot they can do. It is here to stay unfortunately, and foreign actors are employing these strategies more and more.