If you are using Authentik, it will work good, but may be uneccessary if all you want is secure authentication, this is becuase Cloudflare does a "Zero Trust" model in which only approved email addresses, would be able to acess your site which you have to manually approve, otherwise, they get hit with the Cloudflare wall. This is completely free and voluntary!

In terms of Cloudflare, I assume that you have your domain all set up with the DNS servers pointed to Cloudflare from your domain registrar. This is probably the hardest part of it all. Then, it's just a matter of setting up your redirection to your server. I used to use them but haven't been in it for about a year now for my domains. They are a good company and will certainly help with your site's security.

Nginx Proxy Manager is like the reception desk for all traffic, when WWW comes in and asks for site.example.com NPM sees that and matches it up with the IP address/port of your server and sends it on it's merry way. If you are using Cloudflare Tunnels, and after you install their tunnel adaptor, you can skip NPM and just point to the IP and internal port on your server. Pretty straight forward in that area. They have great documentation on how to do this to get you started.

Basically, if you use Cloudflare tunnels, you can skip NPM and free up some resources if you want as it will be redundant. So, then Authentik comes in and can act as a traffic cop and protect all types of pages, ones with log in and those without.