Are any apps available for the Kindle e-book e-reader?

For a Kindle Paperwhite, what apps are generally available? Is there a calendar app?

There are clear limitations with e-ink screens, but, are there not, or were there, apps such as chess, calendar, maybe even e-mail for a Kindle?

https://www.amazon.com/Calendar-Pro/dp/B004Q7GWYG/

looks to be specific to Kindle tablets, and not e-readers?

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(surfing the net on a Kindle is, to my mind, an altogether different topic.)