Windows 10 mobile app store vs windows phone 8.1 app store differences?

I want to upgrade to win10 mobile on my lumia 640 but was wondering if the apps on the two stores are compatible? Do apps on windows phone 8.1 work on windows 10 mobile or can be found on the win10 mobile store?
Are there less apps on the win10 mobile store?

Sorry, I'm just not sure how this works.

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