These symbols are pulled from the standard Unicode ranges. Tap or click any of them to copy, then paste into the Free Fire name change screen, your Instagram bio, or anywhere else that supports Unicode text. There is no font installation, no application, and no platform-specific trick, the same character renders the same way wherever a Unicode-aware font is loaded.
If a symbol appears as a box on your device, the font you are using does not include a glyph for that codepoint. That is not the symbol being broken; it is your local font missing it. Try a different category, or use the symbol on a different device to see whether the rendering changes.
The classic Free Fire pattern is one decorative symbol before the name, one after, and the player's tag in the middle: ꧁ PlayerName ꧂. Doubling up. For example ⚡⚡ on both sides, looks busy in the kill feed; one strong symbol per side reads better in-game. If you are aiming for a couple name or a clan tag, replace the prefix or suffix with a meaningful character (a heart, a flame, a clan letter) rather than stacking decorations.