Why am I getting unsolicited emails from companies whose email address contains “apple mail”?

The sending addresses are spoofed, which is why spammers do that so blocking becomes useless. There are third party spam blockers you can look into, but you also have to make sure they are not blocking legitimate emails.

Whenever you give a company/website your email address, there is a likelihood that they are going to sell it and that is how the spammers get ahold of it. The best method I have found is to use Hide My Email so you can create custom emails addresses to use to give to a service when you sign up. That way you can identify which company is selling your email account and also easily remove that email address so you no longer receive any spam from it.

Create and manage Hide My Email addresses in Settings on iPhone - Apple Support

The downside is that it offers little help to an email address that has already been exposed to spammers and also the Hide My Email service requires an iCloud subscription. That can be as little as 99 cents per month for the additional 50Gb of storage.

You are correct that the Unsubscribe button is only beneficial from a legitimate company where they will remove your account from the mailing list. A spammer has no incentive to do that.

Don't open them.

They probably got or bought your email address and use Apple Mail as a enticer for people to open them.

Apple wouldn't sell your email address. They have a reputation to uphold being a trillion dollar company.