230V switch controlled by Arduino Uno (Atmega 328)
Use a contactor.  Like in the link.

https://www.homedepot.com/p/Noark-40-Amp-2-Pole-Definite-Purpose-Contactor-Ex9CK40A20B7/300812949

These have a 24 volt AC coil and can switch on some big circuits.  I used something like this in one of my own projects.  If you can figure out how to switch a 24 volt AC circuit then you can turn on this contactor.  I'm not sure of the current draw on the coil but it's light enough for common low voltage household thermostats to handle.

In case the link is broken I'll point out that contactors like the one I linked to are apparently used for HVAC control, sprinklers, and heavy duty lighting.  I've seen them rated to 250 volts @ 50 amps.  Most any place that supplies home builders and HVAC will have them.  I suspect the do-it-yourself type of hardware store might not have them, and might not even know what they are if you ask.