Apple runs App Stores for Macintosh computers and for its mobile devices. To download an app from the Mac App Store, you need an Apple ID. Once you have logged in successfully, search for free applications, and you should find some available for installation.
iPhone apps only run on iOS devices, such as an iPhone or iPad. However, many such apps do have desktop equivalents that may behave similarly to the app on your phone, and they may interact seamlessly with the app you have installed on your phone.
Apple does require you to supply a payment method when first registering for an iTunes or App Store account, but it will not be charged unless you buy something that costs money in the App Store. To prevent inadvertently making a purchase, after registration you can change your payment option to "none."
Many Android apps will run on a desktop computer, including many popular free ones. This is because an increasing number of apps can run inside Google's Internet browser, Chrome, whether on a Chromebook or a Macintosh, Windows or Linux computer. Many Windows Phone apps can also run on several platforms, such as on both Windows Phone and a desktop running Windows 8 or above.