Hey Richard, 

I'm not sure i can fully capture the power and capabilities of the mendix platform in this answer, but I'll try and tackle your specific questions –Mendix itself is a PaaS and as such it can work with various platforms and devices, iOS, Android, and primarily responsive web-apps.

-The Mendix marketplace is strictly an ecosystem for the developers to publish reusable content, templates, modules etc – it's unrelated to who can access apps and how they themselves are distributed. 

-No, the users of mendix apps are similar to the users of apps anywhere else – the security is app-dependent. For example, Active Directory, external SSO, OAuth – those are all set up per app on a case-by-case basis. Only the dev team needs mendix accounts. 

-It does not require power apps to use, similar to my answer to the first. Mendix will create apk/ipa files and can connect with various distribution services (i.e. App Center) as needed. It's not used to create desktop applications, but on the responsive web-app front, Mendix can work with all different architectures – Azure, AWS, on-prem, etc. as well as it's own Mendix cloud for hosting applications

I recommend going through the introduction and more features here:

https://www.mendix.com/platform/

• Is this a platform to make apps for any platform?

You can make web applications, native applications, progressive web applications

Does this only make apps for a Mendix based marketplace?

• Nope, it doesn't

Do users of the apps have to have a Mendix account to use the apps?

• Nope, you can use a separate user management system within the applications. The applications are often also built to work with various active directories / single sign on solutions.

Does this create app packages that can be installed on Windows or mobile platforms in .exe and .apk files, or is it strictly a clone of Microsoft Power Apps where you have to use Power Apps in order to use the app you design?

• It creates packages that you can deploy to a cloud environment or on-premise, and for native it creates packages like apks.

As addition to previous great answers;

Mendix is definitly NOT a clone of MS power apps. if one is a clone, its power apps which is a clone of Mendix. Mendix is more powerful then Power apps ;-)

Great answers here. Also see the platform evaluation guide, which covers a lot of topics when you’re exploring the platform:

https://www.mendix.com/evaluation-guide/