What should go in a welcome email (or welcome series)?
The welcome email confirms the signup, delivers the promised value (like the lead magnet) right away, and sets expectations. It is the best-opened email you will ever send; a two-or-three-part welcome series is the highest-returning automation there is.
Nobody is as engaged as the subscriber of five minutes ago: welcome emails get far higher open rates than regular editions. Do not waste that momentum on a bare "thanks for subscribing". The first email arrives immediately, delivers the promised lead magnet, introduces you in a few lines, and sets the expectation: what, how often, and what is in it for the reader.
A series beats a single email. Email two (after two or three days) shares your best existing material: the three articles or editions you are proudest of. Email three (after a week) may ask something: a reply about what the reader struggles with (gold for your content planning) or a soft pointer to your product or service.
Technically this is the simplest automation in existence: trigger on signup, two or three emails with delays in between. MailerLite does it free, and at beehiiv, Kit, and everyone else it is a template job for one evening. No other email investment pays back this fast.