Where in GA4 reports do you find the
ga=? This is popping up on the end of a lot of my URLs.
You don't find it directly in the reports. If you really need it, you can get it in a BQ export. That value is just a few ids needed for customer and session continuity. It's normally used for when users navigate across top-level domains (TLD) which send data into the same analytics property.
GA stores session and client information in a cookie to be able to seamlessly glue different hits to appropriate sessions and users. Cookies can't be shared across TLDs without the use of third party cookies or complex backend support, so the solution Google came up with is to append vital data to the external links when these links contain domains from the cross-domain tracking setting.
You can read more about it here.