Drink less paranoia smoothie…
I’ve been self-hosting for almost a decade now; never bothered with any of the giants. Just a domain pointed at me, and an open port or two. Never had an issue.
Don’t expose anything you don’t share with others; monitor the things you do expose with tools like fail2ban. VPN into the LAN for access to everything else.
127.0.0.1 is the loopback address. Ie it’s your computer trying to reach a service its hosting (prowlarr)/talk to itself. Sonarr/Radarr should be using this address as long as prowlarr+sonarr/radarr are on the same machine.
Prowlarr is either not running, or port 8080 is blocked by your firewall.