url rewriting – What is the difference between these two NGINX rewrite rules and what do they actually do?

I have NGINX reverse proxy code that I’ve been experimenting with (through trial and error) to ultimately get the result I want.

The below rewrite rule does what I want it to do; however, I’m not sure if there’s a better way to do it since I stumbled upon this through trial and error. Also, I don’t want any unexpected results.

rewrite ^/$ /web/index.html;

Originally, I incorrectly thought the below rewrite rule should do what the above rule does.. However, it does something completely different. I would like to know what it actually does in contrast to the first rewrite rule.

rewrite ^/$ http://$http_host/web/index.html;