One mark per letter
Strikethrough here uses U+0336 COMBINING LONG STROKE OVERLAY after every character. Underline uses U+0332 COMBINING LOW LINE. Your letters stay normal letters, so the text is still readable by search and screen readers, which is not true of most other fancy styles.
Why the line has gaps in some apps
Each stroke is drawn by the letter it belongs to, so the line can break between letters or under spaces, especially in narrow fonts. The long stroke (U+0336) gives the most continuous line. The short stroke (U+0335) is subtler.
Want every style at once? The fancy text generator casts all of them from one box, and how Unicode fonts work shows what’s going on under the hood.