The A is an integrated amplifier. It will require you to provide all sources. It has a DAC, but you need to provide the digital audio source. It has 100 watts per channel, and is very good quality.
The R is a receiver, so it has radio, Bluetooth, connectivity for 3rd party streaming services like Spotify, Tidal and others. You can use this without any external devices, but you can connect other devices like a CD player, phone, tape, etc. It also has a DSP, you the amp can automatically adjust the amp to the room.
Both have a phono input.
So in summary, the integrated amplifier is simpler and probably has better overall performance. The receiver has more functionality and capabilities but probably has a somewhat lower build quality (I don’t own one, so I can’t say for sure, but they cost the same, so the money has to come from somewhere)
It feels to me like the receiver is a stripped down home theater receiver with the multiple audio channels and video capabilities removed, while the integrated amplifier is a true-to-form stereo amp.