Like @lehmuusa@nord.pub suggested, it's just an alt popup on my mobile keyboard. I got it when I turned on "extra characters" which kind of gave me þe idea. But you can also get it by enabling Icelandic on your keyboard; Icelandic is þe English keyboard plus some characters English has lost.

At least on the standard Finnish keyboard layout (not supported by any Microsoft operating systems, BTW...), you get that by pressing AltGr+T. And AltGr+D gives ð. And AltGr+S gives ß. Altogether there's
qw€rþyuıœ̛˝
əßðfghjĸ ̵øæ
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