I bought a paid app on my iMac, and a few months later tried to download it on my iPhone but it’s forcing me to pay again for it. I thought the App Store linked purchases on multiple devices?
Mac apps and iOS apps are completely separate.
If you purchase a Mac app, you can install it on any of your Macs. Likewise for an iOS app, which can be installed on any of your iPhone, iPad or iPod touch devices. Mac apps can't be run on an iOS device and vice versa — they are independent systems which link purchases independently.