A vs CNAME

Domain name is parent of ip address
domain name : ip addr = 1:N

$ dig yahoo.com

; <<>> DiG 9.14.8 <<>> yahoo.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46812
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;yahoo.com.                     IN      A

;; ANSWER SECTION:
yahoo.com.              953     IN      A       98.138.219.232
yahoo.com.              953     IN      A       98.138.219.231
yahoo.com.              953     IN      A       72.30.35.9
yahoo.com.              953     IN      A       98.137.246.8
yahoo.com.              953     IN      A       72.30.35.10
yahoo.com.              953     IN      A       98.137.246.7

;; Query time: 12 msec
;; SERVER: 192.168.0.1#53(192.168.0.1)
;; WHEN: 火 12月 31 22:21:38 JST 2019
;; MSG SIZE  rcvd: 134

Difference between A Record and CNAM

record typemeaning
A(Address)Domain Name to Ip Addr
CNAME (Canonical name)Domain Name to Domain name

https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/