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:
You can make web applications, native applications, progressive web applications
Does this only make apps for a Mendix based marketplace?
Do users of the apps have to have a Mendix account to use the apps?
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?
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: