It is the letter D rendered as the Unicode character 𝔇 (code point U+1D507), part of the Gothic Fraktur styled alphabet. It is a single character you can copy and paste anywhere.
How do I type 𝔇 on my phone or computer?
You do not type it from a keyboard; you copy it. Press the copy button above and paste it into any app. That is the whole point of Unicode styled letters.
Where can I use the Gothic Fraktur D?
Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Gothic Fraktur alphabet is linked above.