Great question — answering as the UphillTech team:
You don't have to create a new stream. Since you are dealing with a subdomain, it is perfectly reasonable to have one stream for both sites. If you are dealing with a situation where you have different site owners, then it might make sense to have separate streams for your subdomains. Each stream will have their own measurement IDs. I believe each of these will also be dimensions in the GA4 property where you can create audiences and reports for specific sites. The only thing you might need to adjust is the referral settings in the stream. you might also want to push a new parameter to the tag for the subdomain to allow for easier reporting to avoid any potential issues using the domain or...
Full answer by A_Patterson: https://webmasters.stackexchange.com/a/143684 (CC BY-SA 4.0).
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