This app was mentioned in 2 comments, with an average of 3.00 upvotes
did some digging:
http://www.kaligrafina.com/samsungtaba-spen-handlettering/
thats from the op, you can translate that page (sorta).
i believe he is using this app VVV called calligrapher.
he is also using the autodesk sketchbook app
https://play.google.com/store/apps/details?id=com.mark.calligraphy&hl=en
It's called Calligrapher. I have a Samsung Note 10 which comes with a stylus so it's definitely easier to write on the phone. It's from the Play store so not sure if iPhone users can download it. https://play.google.com/store/apps/details?id=com.mark.calligraphy