I use (Apple) Notes on my iPhone and MacBook. They are always in sync. The notes don’t use MarkDown, but they are plenty of styles and features. I highly recomm

I use (Apple) Notes on my iPhone and MacBook. They are always in sync. The notes don’t use MarkDown, but they are plenty of styles and features. I highly recommend checking (Apple) Notes out. I’ve used MS OneNote, which is also very good and available and syncable on/across several platforms. For me, (Apple) Notes has a slightly better UI. If you just want a cross platform syncable to-do list, and the above are too heavy, you might look into MS ToDo 2.0.