To start off, I'm a noob. I'm using github and CF Pages to host a webpage, I also have a custom domain. Another company asked for our server's IP to configure their API connection. I'm guessing it's some kind of whitelist. This API would be for me to fetch data from them. As I understand it, CF Pages IPs aren't necessarily static. When I do an nslookup in terminal I get that my site's IP is 188.114.96.13 and 188.114.97.13. When I use some third party IP finder I get that the IPs are 172.67.203.182 and 104.21.44.210, and I do seem to have A records pointing my root and www.root to these last 2 IPs. All of these are CF IPs, but they aren't all on the https://www.cloudflare.com/en-gb/ips/ CF IP tables. So my question now is, how does this work? Why are different IPs being detected? And what IP or IPs am I supposed to be giving them, seeing as my website's IP could change?

Those are all ingress anycast IPs. CF is not a simple server host where it has one IP for inbound/outbound. How are you fetching data from them? Using Cloudflare Functions/Workers? The IP Ranges list you linked is the IPs Cloudflare uses for egress from Proxy records/Workers (anything else - ingress IPs etc, is not included). Any of the IPs in that range could be used. It may be easiest for you to use a simple Virtual Private Server with a static ip to fetch from them, if they mandate using an IP Whitelist, as otherwise they'd need to whitelist all of Cloudflare, and none of those IPs are exclusive to just you

I am not fetching anything from them yet. They asked for the IP for setup. I guess I'll explain that I'm using CF Pages and they'd have to whitelist every other page hosted through CF Pages too and we can come up with a solution.