Here are the exact Flux steps for your three items, quoted from the docs.
Fills can be enabled or disabled on each layer by assigning a "Connected Layers" rule to the target net. If the target net is a ground net it is known as a ground fill, while if the target net is a power net it's known as a power fill.
Helpful context on ground fills:
“When you add a GND symbol, Flux will auto-generate a ground fill. Adding a GND symbol will generate the following:
I could not find documentation that explains a manual, separate “ERC run” operation for schematics. This probably means there isn’t a dedicated manual ERC trigger in Flux. DRC runs continuously, and AI Design Reviews are available for additional checks:
“Finalizing Layout: Let DRCs run automatically with each layout update, addressing physical rule checks to ensure manufacturing readiness.”
Source