Using an Apple Watch Ultra in extreme environments...

I haven't experienced any problems using my Ultra watch during my alternating sauna and cold plunge sessions. However, I would like to inquire about its warranty and operating temperatures. Additionally, I'm curious about which workout app to use for cold plunges (48 degrees) and saunas (141 degrees inferred). I'm starting to question if this watch is truly durable and rugged enough for my needs based on some responses on the line or the interwebs.