Selfhosted

It's about sample size. Mail gateways won't designate an IP as a reputable sending IP until it assesses a large volume of mail sent over a long period of time. You can't send the quantity it wants all at once or even in a short window because then you'll be designated as a spammer. So you start small with a few a day and gradually ramp up sending over multiple weeks or months to eventually send several thousands of messages in that period.

Spammers and malicious actors too often spin up new IPs for sending mail, so gateway patterns already implicitly mandate that email should come from IPs it's already judged reputable.

You as an individual can't reasonably warm your own IP. This is why services like Amazon SES or Sendgrid exist because they have huge IP pools that are ready to go. Plus, those services are very concerned with reputation and have bounce/complaint metrics defined to warn customers that abuse or poorly configure their sending habits.

This next example is what I'm most familiar with, but I'm sure there are other services like this. If you're a big enterprise and want your own dedicated sending IP because you're concerned about using a shared pool, you could use something like Amazon Pinpoint which allocate IPs for your org to use in SES, but they have to be warmed before you switch your production workloads over to it full-time. It automates some of the gradual-ness of warming so you use a mix of SES plus your Pinpoint IPs to keep mail flowing for your product.

It looks like Sendgrid also does dedicated IP warming guard rails too. This article is pretty decent for understanding how it works - https://docs.sendgrid.com/ui/sending-email/warming-up-an-ip-address The per-day warming limits give you an idea of what scale this kind of process is used for.