Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Upside Down alphabet is linked above.
What is the Upside Down letter B?
It is the letter B rendered as the Unicode character q (code point U+71), part of the Upside Down styled alphabet. It is a single character you can copy and paste anywhere.
How do I type q 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.