I found a 30W USB-C charger plug in my drawer. I think I must have picked it up around 2019. I bought a lot of USB-C to X(other connector type) Apple dongles back then. I have a USB-C MacbookPro and iPad so maybe it came with the iPad. I don't remember. I use a big brick to charge both.
Any harm in me using either the big brick that came with the MacbookPro or the 30W that I found in the drawer? Both Apple. I don't want to limbo under my desk and out the other side to look at the big brick unless it's important.
What IS the optimal charger to use if I have to buy a new one tom preserve the iPhone 15's battery health?
There isn’t one truthfully. Their power IC handles voltages / wattage.
If you are really concerned about battery health, turn on the new “80% charge limit” option. This option will preserve your battery health the most.
Other ways to preserve battery health is to charge the phone properly. This consists of not always fast charging it, not having it plugged in all the time, etc. There’s a whole list that you can find online.
For reference: I charge my phone(14 PM) 99% of the time via MagSafe. Every night I drop my phone on the charger and every morning I take it off. My phone is off the charger for about 14-16 hours a day. If I catch myself running out of battery, I throw it in low power mode early enough that it’ll last me all day. When I’m traveling, I have a 20w USB-C and USB-A charger that charges my watch and phone. I use this charger during the day to top off my phone for the night and then I put my phone back on the charger when I go to bed.
Doing all of this, my battery health is 98% after a full year of MagSafe charging with about 40 corded charges.