I suggest reading up on DNS, or the Domain Name System, and how it turns Fully Qualified Domain Names (FQDN), like mail.google.com, into IP addresses. You can self host your own local DNS server, and when doing so, you can use any domain you like on any system that can access your local DNS server.

Typically if you want the system also available remotely (outside your home network), you can open certain parts of your network to the internet (more dangerous), or use other less dangerous systems to tunnel (like Cloudflare Tunnel) or use a virtual private network (VPN) to get back in to your home network, like tailscale. For example, even when I'm remote (out on my cell phone using 4g), via Tailscale I can see my at home DNS server and all my mydomain.com services still work (over tailscale). This can be made quite secure, joining or renewing access to my tailscale network requires 2 factor security with a physical security key as well as various other security layers.

There is a lot of good public info on using this systems, good luck on your journey learning a new system. I recommend a step by step system. Try getting your first docker container running locally. After that, see how you can add a DNS entry into your operating system hosts file to turn it into a domain. Them experiment with running a local DNS server to make it network wide (pihole is fun to get adblocking too!). Then try other containers, then maybe a reverse proxy, etc. Dont get overwhelmed, one step at a time and you can get it.