Sending html emails. How exactly does it work?

Question

PHP

Hello. I'm at the level of html letters... zero, approximately. Never dealt with them. And then they set the task - to make sure that the manager from the admin panel could send them with one click. Those. make a template, by clicking on "send" insert the data from the form there and send it to a known email address

Everything would be fine, but I have a rather poor understanding of how this system works in general. Well, I'll write like this, for example ... and what will the recipient receive? Html file or expanded email? And what will happen in this particular case with the display of pictures? In Runet, I saw little information and mostly it is old.

I would really appreciate your advice :)