I have some subdomains that go to my home address (I know I should put it through a VPS first but I’ll get to that when I have time).
If I connect to example.domain.tld and DNS records point back to my own IP, where does that data go to reach back to my device?
Devices on your domain will typically do a DNS lookup, which gets your public IP. Then they connect to that public IP, which your router recognises and redirects back into your network. The router then forwards that to your reverse proxy.
If your router isnt doing that properly (timing out usually), look up a setting usually called “NAT loopback” or “NAT hairpinning”. Thats the setting that detects your public IP, and redirects it back inward.
Also if you have your own net filter like PiHole or AdGuard Home with DNS rewrites set up, and you use it as a DNS server in WiFi settings, then it works a small bit differently. (I need to do this because my current ISP removed their DNS settings page for the new model). A public IP and NAT routing is never needed, as the device contacts the DNS server via the router Access Point, and the DNS server translates the service’s FQDN into its internal IP. Aside from that, provided everything is set up correctly, all actual data packets go from device ←→ router ←→ service. If the router lost connection to the Internet this wouldn’t break communications.
I had issues with that recently, I had a few of my internal services set to resolve internally, but pihole was making a mess of them and returning IPv6 addresses in addition to the IPv4 internal addresses. And then browsers would try use the broken ipv6 address and fail. I just happily rely on hairpinning now, it hardly makes a difference in the scheme of things.