Spam is often sent from big providers, too. For several years I hosted my email the “middle ground” way (i.e., relaying outgoing mail via Google Workspace), and despite using DMARC correctly it was not infrequent that my emails would go to spam (even to GMail boxes) or never show up at all.

Obviously GMail is such a giant that email providers have to be very careful when blocking it, but enough spam comes from there that receivers clearly use some heuristics to block some of it. I’ve even received multiple rejection notices because the GMail server my email was sent through happened to be on a blacklist!

I switched last year to sending directly from my VPS. It was partly for privacy from Google, but moreso so I could enforce outgoing TLS. For the first few days they went to spam boxes or moderation queues, but I made sure they were rescued, and ever since I’ve had no deliverability issues sending to Google, some local ISPs, and even Microsoft (which seems crazy, as I never got a mail from my domain to show up in Outlook when I was relaying through Google).

I can only speak for my own experience, of course. But that is what I experienced.