Can you explain how my external IP works with these nftable rules?

Question

My server (under my control) is connected to a Cisco router (not under my control)
It is Debian linux server
My Server IP: 10.119.248.14 (in /etc/network/interfaces)
My ISP have given me 14.139.114.115 as my external IP Then ISP asked me to do natting.
With trial and error I found that following two rules established access to internet/ hosting website etc
/etc/nftables.conf

chain prerouting {
                type nat hook prerouting priority -100; policy accept;
                ...
                ...
                iifname "eno1" dnat to 10.119.248.14
        }

chain POSTROUTING {
                type nat hook postrouting priority 100; policy accept;
                ...
                ...
                oifname "eno1"  snat to 14.139.114.115
        }

Now I can access my website with 11.239.114.115 from outside
My questions are

1. If incoming packet received do not have 10.119.248.14 ip, then why my server is accepting it in first place?\
2. What type of configuration my ISP may have done, so that, to external world I am 14.139.114.115?