Do you already have a navigo easy card from a previous trip? If not, I believe you can buy a card and load it with both metro and bus tickets since they are now separate prices. We had an older card that still had the T+ tickets on them, they were good for both, so I couldn't add additional tickets of either type until I used the T+ tickets (original navigo purchased and loaded Nov 2024, used T+ tickets for both metro and bus April 2026). You can also use the IDF Mobiles app to check balance of cards and/or add tickets (with some restrictions on mixing types, i.e. we couldn't add anything until we used our old ones), as long as your phone has NFC on. If you're in the US, with US telephone you can't actually set up a personal account but you don't need one to check/add to your navigo card with the app.
Alternatively you can buy a ticket directly through the app, and use that to board bus and metro (check me on that, I know we did it on buses). You can also use the SNCF-connect app to buy a ticket on your phone.