Replacing iPhone battery feels like reassembling a mechanical watch. It isn’t soldered, but it’s glued in weird way requiring applying heat. To change battery, you have to disconnect 4-5 tiniest connectors, looking like LPT for fleas, extremely fragile.
https://www.ifixit.com/Guide/iPhone+12+Battery+Replacement/1...
And you just can’t buy original battery, they’re available only to selected service centres.