In order to have the same style, you have to copy the scss file or add “,ion-custom-segment” where ion-segment is use in scss.
A quick replace in scss will do the job
In other hand, some ionic component have custom class after treating, maybe if you look on computed html a class is added to ion-segment and you just have to add it in your own tag.
I don’t understand the second part of your answer " or add “,ion-custom-segment” where ion-segment is use in scss".
Can you make a quick example please.