Port 5353: mDNS
Port 5353 is multicast DNS, the way Apple Bonjour and similar systems find devices on a local network without a DNS server.
What runs on port 5353
Devices send and answer DNS-style queries to a multicast address on UDP 5353 to advertise names like printer.local and services like AirPlay or AirPrint. Apple calls it Bonjour, Linux uses Avahi, and Windows 10 and later support it. It is designed to stay on the local network.
On a home network
Almost everything on a modern home network uses it: iPhones, Macs, Apple TVs, HomePods, Chromecasts, Sonos speakers, printers, NAS boxes and smart-home hubs. Seeing 5353 is completely normal inside.
If port 5353 is reachable from the internet
mDNS should never be answered from the internet. Some devices have been found replying to mDNS queries from outside, which leaks device names, models and services and can be abused for reflection attacks. On the LAN it is harmless, though it does tell anyone on the same Wi-Fi a lot about your devices.
How to check whether it is open on your network
A Bonjour browser on a phone or laptop lists everything advertising over mDNS on your network. An outside-in probe of UDP 5353 on your public address should get no reply.
To find out whether port 5353 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.