Brad Linder of Liliputing published an article titled How to use laptop or tablet as a touchpad (and keyboard) for a Windows PC. He turned an Amazon Fire Max 11 tablet into a touchpad and keyboard using a proprietary app called Unified Remote (which also required setting up Google Play Store on the Fire tablet). I was surprised he did not mention my preferred solution. KDE Connect is free, open source, and can allow Android devices to function as touchpads and keyboards (I previously covered some of my favorite open source Android apps). I seldom use it for that purpose (my main KDE Connect use-case is sending and receiving SMS messages from my computer), but it works well. Moreover, people with Fire tablets can install KDE Connect without needing to mess around with Google Play Store.

I will note in a small aside that I have only used KDE Connect on Linux PCs. While I understand that it is available for Windows, I am not certain that it works exactly the same as on Linux.