watermelon0 has the correct answer: [most] hobbyists should stick 100% to low-voltage low-current applications and use off the shelf certified hardware whene interfacing with high-voltage (i.e. mains) power.

MEE hobbyists, disregard.

Even with a MEE your Homeowner's policy may deny a claim due to non-UL listed components installed along the mains.

I won't touch electrical on my house and will happily contract it out to a license+bonded electrician for the exact same reason.

That said really cool project and Kudos for tacking it, just make sure your smoke detector is working :).

If you have a device drawing high current, you would probably use a contactor (high-power relay), and then you can use low current (or even low voltage) relay/triac to drive the contactor from your circuit.

High power relays with safety certifications are typically used in mains boxes available commercially. In fact, I'm using the ones from my old mains box.

Even theoretically, the only way a relay could catch fire if the coil inside overheats or if you pass very high voltages to the coil. But, with a well designed circuit, it should be fine.