Effectively: what is the difference between some-form-of-buildpacks and Cloudron's current packaging format?

Good question. You should find the information you need in https://docs.cloudron.io/packaging/manifest/

For context: I've been running and creating Heroku buildpacks since they launched 10+ years ago, precisely because buildpacks are simpler than Docker and make things easier for mortals to self host

Now, thinking about a broader "app store", and more hands maintaining common apps, I'm wondering about connecting with other communities to make lighter maintenance work.

Effectively: what is the difference between some-form-of-buildpacks and Cloudron's current packaging format?

Do I take this to mean that I can take any apps's buildpack, say from Heroku (or from any of those listed), and just run it in any of those mentioned companies, and it will just work?

Yes. There is a growing list of platforms that support Buildpacks. There are some related efforts like nixpacks growing as well.

eg. here's Digital Ocean list of buildpacks they support https://docs.digitalocean.com/products/app-platform/reference/buildpacks/

https://buildpacks.io/

The website says that buildpacks has "been adopted by Cloud Foundry and other PaaS such as Google App Engine, Gitlab, Knative, Deis, Dokku, and Drie." Do I take this to mean that I can take any apps's buildpack, say from Heroku (or from any of those listed), and just run it in any of those mentioned companies, and it will just work? If so, that is impressive and very useful. But I also have doubts that it is actually that transferable- surely there is some tweaking of options or env details needed? And if so, then I wouldn't apply the label "standard" to it.

Perhaps you need to see the BayLISA talks about the origins and architecture of Cloudron.

See https://videos.cloudron.io/w/51jcnUxgd2JetwQ2XAeyWM