Varnish cache and built-in cache are both caching solutions that can be used to improve the performance of Magento 2 websites. However, there are some key differences between the two.
Varnish cache is a reverse proxy server that sits in front of your Magento 2 website and caches static content, such as HTML, CSS, and JavaScript files. This can significantly improve the performance of your website by reducing the number of requests that need to be made to your Magento 2 server.
Built-in cache is a caching solution that is built into Magento 2. It can cache a variety of content, including static content, dynamic content, and database queries. However, built-in cache is not as efficient as Varnish cache for caching static content.
Which cache should use?
If your Magento 2 website is heavily reliant on static content, such as images and product pages, then Varnish cache is the better option. Varnish cache will significantly improve the performance of your website by caching this static content.
If your Magento 2 website is not heavily reliant on static content, or if you are not comfortable configuring Varnish cache, then built-in cache is a good option. Built-in cache is easy to configure and can still provide a significant performance improvement.