For those who prefer to buy their bus and metro tickets directly on their iPhone, there’s great info here on to do it from Man in Seat 61. It was very easy to buy tix using my iPhone’s Apple Wallet. This info applies to iPhones. Read more at the Man in Seat 61 link below, under the heading “How to buy metro tickets”
https://www.seat61.com/changing-stations-in-paris.htm
HOWEVER the tricky bit is you have to allow for “express transit” within your preferences in Apple Wallet to sort of “activate” or “wake up” the ticket before its use at the metro turnstile or aboard the bus’s scanner. When you open your Apple wallet and see your Paris metro/bus ticket sitting there, click on it. Then click on the top right corner where the circle and dots are and click on “card details”. Click on “enable express transit.” You’ll notice each ticket (if you buy more than one) has a unique assigned number, so you’ll need to make sure to activate the right one.
Info from RATP here: https://www.bonjour-ratp.fr/en/aide-contact/?question=what-is-the-purpose-of-the-express-transit-mode-on-iphone-or-apple-watch
My recommendation would be to buy one bus ticket, practice activating it as outlined above, and ride the bus once to make sure you have it down pat and build some confidence. :)
This took me about three days to figure out and a lot of “je suis très desolées” to bus drivers when my bus ticket would read as invalid with a red X. They didn’t know how to help me!
Once you have the express transit setting on, you just wave you phone on the scanner and waltz on through. You don’t even have to have Apple wallet or the ticket open.