How does composer autoload files work

Hello all, I downloaded a library (dompdf) on github.com to covert html to pdf. According to the instruction, I was told I could use composer to install the library in my project (this is not a laravel project). But I didn't I only included and required some files) and it worked perfectly in the test environment . But when I moved to production. I noticed that the file autoload.php from the library was conflicting with a class in my project. The error says that it Couldn't find that class) Why does the autoload affect my project classes.