It is the letter J rendered as the Unicode character ſ (code point U+17F), part of the Upside Down styled alphabet. It is a single character you can copy and paste anywhere.
How do I type ſ 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 Upside Down J?
Anywhere text is accepted: Instagram bios, TikTok names, Discord, WhatsApp, gaming profiles and documents. The full Upside Down alphabet is linked above.