Google's John Mueller said when it comes to crawl budget, it is possible that Google may group your subdomains together for crawl budget purposes. John explained "our systems try to figure out which infrastructure belongs together."
Some may have thought Google will have different crawl budget allocations per domain, subdomain and so forth. But it really depends on the site, Google may or may not group all your subdomains on the same domain for crawl budget purposes.
Here are those tweets:
It's possible we're already grouping the subdomains together for crawl budget -- our systems try to figure out which infrastructure belongs together. I suspect you would see some crawling fluctuations there, but that settles down automatically as our systems figure it out.
— 🍌 John 🍌 (@JohnMu) March 31, 2021
Forum discussion at Twitter.