On my new Iphone 15-Actually once you have done everything above and your yahoo mail is set up on your phone with the calendar tab "on", you have to open the yahoo app, open calendar full view, click the actions arrow, pick share, and your main calendar will be in the block(or arrow down key and pick the calendar you want to share), click share button, scroll down and turn on "pick shareable link", copy the link. Now on your iphone-open your calendar and at the bottom click "calendars" a small box will open, pick add subscription calendar, paste the link in the subscription url box. it takes about 5 mins for in to start pushing the yahoo data into your phone, but it will work.
If you just want to add the data sync without adding yahoo to your phone-when you are in your main iphone settings pick other instead of Yahoo! open your iphone calendar and at the bottom click "calendars" a small box will open, pick add subscription calendar, paste the link in the subscription url box. it takes about 5 mins for in to start pushing the yahoo data into your phone, but it will work.