Developer experience is a product
The most important feature of an internal developer platform is that the team that builds it has to compete to win over their users.
Figure out your initial value proposition, build a minimum viable product, get it in front of customers, listen, learn, and iterate.
Platforms imposed by a top-down mandate tend to fail.

Over the past 15 years, I've been working on one form or another of internal developer platform. Even long before, while working at small startups, I inevitably ended up building (or curating) some little web framework, a build system, and slapping together scripts to package and deploy our stuff reliably. No one ever told me to do this, it was just obviously necessary.
In these cases, I was building a product for myself and my immediate team members, so it was a pretty tight feedback loop with the customer. I'd put a little extra effort to make things nice for other developers on my team, and also out of a bit of pride in making something that felt elegant.