Google analytics plug-in : analytics.js vs gtag.js

I’m sorry; I just came across this post (https://forum.pkp.sfu.ca/t/google-analytics-4-in-ojs3-3-x) right after I submitted mine (I don’t understand why my preliminary search didn’t find it).

Am I to understand that once a GT code is obtained (this would be the first step then), I can simply modify the lines (Google Analytics 4 in OJS3.3.x - #5 by yasielpv)? Do the UA code and the GT code are meant to live together or once updated/remplaced, only the GT code remain?

Many thanks,

Marie-Hélène V.