I use my right hand for everything. I just can't do anything remotely precise with my left hand. I can't even write with it. The only activities I can do with my left hand are assisting my right hand (for example holding somerhing long).
That's the reason for me as well. Always had wallet and keys in my right pocket, then had to find a place to put my phone when they became a thing. So it ended up in the left pocket.
Same except keys are clipped on a belt loop and I have a handkerchief with my wallet in my right front pocket (Japan is super humid so a lot of sweat-wiping happens). Also right-handed, but I hold my phone with my left and enter the passcode with my right hand (I don't use the fingerprint scanner)