I've successfully have replaced batteries and displays in older iphones (mainly iphone 6). But with newer iphones opening the phone is more complex.

I've read online and heard from Apple Store representative that iPhone 12 (all models) has tendency to crack the screen when phone is opened for repair or battery replacement and in that case Apple Store would replace the hole phone (this is were multi day repair process). So I would rather pay $90 to Apple that guarantees that I'll get a phone replacement in case when screen is broken during battery replacement. Without the phone I sill would be able to answer the cell calls from Apple Watch and with ipad over WiFi.