How are conflicts avoided when using JS libraries

I am currently working through the developer documentation. The docs tell:

“Custom cards are defined as a custom element. It’s up to you to decide how to render your DOM inside your element. You can use Polymer, Angular, Preact or any other popular framework (except for React – more info on React here).”

Do I have to include this libraries myself or are the all available be default? If I have to include them myself, how do I avoid conflicts with other cards including the same library?

Thank you