My paper was accepted in one of Elsevier's journal and it was recently published, but there is no pagination instead they have this "article number" for example "Volume 123, Article No. 212021" similar to those in some Springer and Royal Society journals. The paper is already included in an issue. I am wondering, does my paper officially published or not? Could anyone tell me how this policy is called? or where I can locate this policy in the Elsevier website?
Page numbers are going to go away because (i) stuff isn't actually printed any more, and (ii) page numbers are inconvenient in the publishing process because one needs to know how many pages the articles before the current one have before giving the current one page numbers. So the short answer is: You'll need to learn to cope with it.
In similar situations, I've used the following formats to indicate how many pages an article has:
W. Bangerth: "Stuff more scientists ought to talk about", Journal of Important Research, article 12687 (24 pages), 2019.
W. Bangerth: "Stuff more scientists ought to talk about", Journal of Important Research, vol. 24, pp. 12687/1-24, 2019.