Why does "what's my IP" show a strange / random IP and not my home fibre IP? When I try ifconfig.co or whatismyip.com on my MacOS 10.4 I get a completely unknown IP address and it even changes every day or two.

Tried from Safari, Brave, Firefox and even did curl ifconfig.co from the terminal and all show the same IP, currently somewhere in the US (and I'm nowhere near the US) but as I said it changes occasionally to other IPs.

I don't have any proxies set (checked both in Network settings, and in Firefox), the default route shown by netstat -rn points to my home router, but still somehow my traffic gets routed through these weird IP addresses.

I'm not aware of any VPN running either.

What's causing it? What should I check?
Usually in a typical home setup, you'll have "internal" IP-addresses on your home network. Technically known as RFC1918-addresses, they look like this:

10.0.0.0 - 10.255.255.255
  172.16.0.0 - 172.31.255.255
  192.168.0.0 - 192.168.255.255

These addresses are only used on your local network in your house, or possible also in a smaller part of your provider's network.

Your internet provider uses various forms for address translation to convert these to globally routable (i.e. useable on the Internet) addresses. Therefore the address others on the Internet perceive as yours is not the same as you see configured on your local computer.

Regarding the country information there's two major sources of errors there:

Firstly, the actual determination of which country an IP address "represents" is not accurate, but based on various information published by providers for other reasons. Therefore it might be you're in for example France, but your IP-address is listed as Belgian, because the internet provider you have has their headquarters in Belgium and uses that postal address for their IP-registration.

Secondly, your traffic might leave your provider's network at different locations depending on destination, time of day, how much capacity is currently used by others, etc. Therefore if you look at traceroutes, it might look as if your traffic "appears" to originate in the US or in Japan, or other places even when you're nowhere near those places.