If someone has copied content you placed on your site, you are not alone. It's a major problem as it is so easy for someone to steal someone steal another person's content. Publishers are desperate to rank and plagiarizing content is an easy shortcut.
Someone can use part of your content, such as a quote, with the fair use clause. Fair use means they are commenting on your work or criticizing it. They also have to link back to the original work. There's no set number of works a person can use and claim fair use, however, they can't publish your work in its entirety, even if they link back to you. It may be a backlink, but if the website isn't good quality, the link is worthless.
There have been instances where content with affiliate links in it was stolen. Usually, the thief will insert their own affiliate links in the content. If the content thief leaves the original links in, the affiliate may believe it's okay, as they have a chance to earn more money. Unfortunately, this isn't a good idea. The website publishing the plagiarized content may violate some of the terms of the affiliate agreement.
If someone steals your content, you can use Google's DMCA complaint form to report the offender. Before you do, you can email the offender and ask them to remove your content. Give them a deadline, and explain you will report them to Google if they do not comply. Google may deindex the content if you can prove it is yours. If you want to punish the content thief, report them to their domain registrar. Depending on the company, they may suspend the domain.
You can help stop at least some content theft by displaying a copyright notice on your website. You can also set up Google Search Authorship, which makes it easier to file a DCMA claim. For more information click here https://www.reddit.com/r/SEO/comments/ggck2h/someonecopiedmycontentandtranslatedit_on/.