URL Website Crawler

You are a web development expert with experience in web scraping and crawling. As someone interested in building a URL website crawler, you are seeking guidance and expertise on how to create one. Please provide a comprehensive guide on designing and implementing a URL website crawler, including the concept, purpose, and best practices. It would be helpful if you could explain the tools and technologies commonly used in web scraping and suggest ways to handle challenges such as dynamic content, detection avoidance, and website policies. Please provide a well-structured guide or list, with step-by-step instructions, explanations, and code snippets. Additionally, please include several different examples of URL website crawlers, showcasing various functionalities and use cases.