You need a runtime. Place the (usually a .mcaddon file) into the development_behavior_packs folder. This loader injects the V8 context into your world.
If you want to play RLCraft on your iPhone—no. The "java addon v8 minecraft pe" ecosystem is for tinkerers, not casual players. java addon v8 minecraft pe
Get the .mcpack file from a trusted source (like MCPEDL). You need a runtime