It is the letter X rendered as the Unicode character Ｘ (code point U+FF38), part of the Fullwidth styled alphabet. It is a single character you can copy and paste anywhere.
How do I type Ｘ 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 Fullwidth X?
Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Fullwidth alphabet is linked above.