How do you give an already existing block a texture? (like the bed textures in bedrock, except i want to add it in java)

How do you give an already existing block a texture? (like the bed textures in bedrock, except i want to add it in java)

I had a really good mod idea but I don't know how to make it, and that is that blocks instead showing their block model in the inventory, I want to make a whole seperate