Similar to SEO, there's a lot that isnt public (some for obvious reqsons), so it's a lot of guess work / trial and error / anecdotes. This volume thing I'm pretty sure is real. What is almost certainly real as well is open rates. If you send a bunch of mail that isn't opened, this isn't good either.

The warming up was in the docs for the 3rd party mail service I managed for work a few years ago when we wanted to switch to a dedicated IP. They also cautioned to keep open rates up. I assume they have the data to advise their customers appropriately.

I've mostly run my own mail servers since around 2000, and I gave up a few years ago and started using a 3rd party for outbound SMTP. I had considered giving people free SMTP accounts to boost legit traffic, but I didn't know how to prevent spam/scammers from using it. Like if I posted on Reddit that I was doing that, I'd probably get legit people, but also almost certainly a spammer or few. As such, idk how anyone can practically run their own SMTP server today unless they sort of bootstrap it with a few legit newsletters (that people actually want and open) spread out over multiple days or transactional emails like say a ticketing system (if the people receiving them are the types to actually open them).

As far as personal emails going through the same spam filters, there are some headers newsletters add that I'd assume handles them slightly differently (list-unsubscribe).