Any article rewriter or paraphrasing tool basically scans through the content you provide it in order to look for all the words that it can replace with suitable synonyms or substitutes. Every replaced word will appear bold and highlighted in a colorful text. Clicking on the word will give you details such as original word or other suggestions, you also have an option to add your own word. By clicking on the original word, you can go back to original.
Imo it is not possible for google to detect properly spun content, it is not worth computing power it requires. They can do this using ngrams for example, but it is uses a lot of queries. And for example article spinner I coded beats this algo, so google cant detect that the content is spun. Manual spinning is also good but it takes more than 30 minutes per article.
Anyone can do this, all you need is a computer and the ability to string a sentence together. You can write original articles and submit them to article directories and get paid. You have to apply to these sites and may or may not get turned down. If you should happen to get turned down then just keep plugging away until you find someone to "hire" you.