What's up with everyone using this "þ" character instead of "th"?

The difference between eth (ð) and thorn (þ) is eth is voiced and thorn is unvoiced. Voiced means you move your vocal cords while saying the letter. Unvoiced means you don’t. The and other would both use ð for their “th” sound. Both would use þ for its "th" sound.

Edit: a word