The A7III has a 24mp sensor with 693 autofocus points.
The A7RIII has a 42 mp sensor with 425 autofocus points.
The A7III may perform slightly better in low light at higher ISOs than the A7RIII.
The A7RIII has pixel-shift multi-shoot mode for added sharpness and detail when shooting still objects, etc.
If you need the ultimate in resolution and plan to create very large photos, the A7RIII has the pixels to do so.
If you aren't interested in the ultra-high image size but want a very versatile camera, the A7III is amazing.