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 Bubble (Circled) D?
Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Bubble (Circled) alphabet is linked above.
What is the Bubble (Circled) letter D?
It is the letter D rendered as the Unicode character Ⓓ (code point U+24B9), part of the Bubble (Circled) styled alphabet. It is a single character you can copy and paste anywhere.