It is the letter M rendered as the Unicode character Ⓜ (code point U+24C2), part of the Bubble (Circled) styled alphabet. It is a single character you can copy and paste anywhere.
How do I type Ⓜ 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 Bubble (Circled) M?
Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Bubble (Circled) alphabet is linked above.