The program and assets are stored in a ROM cartridge, so only mutable data needs RAM.
Actually drawing things on screen depends on two things:
The first is racing the beam. The display output traces across the entire screen at 60Hz, one scanline at a time. At no point does a complete image exist, instead you just make sure to apply changes so that the machine can draw what needs to be drawn just before the beam traces that part of the image. You will need to cycle count so that your program takes exactly the right time to execute every section, because you certainly won't have time for interrupts or other synchronization.
The second is using dedicated hardware, where you store the location on screen, color and memory address of a sprite, and the hardware draws it for you. There are a very limited amount of sprites available, which limits the amount of things that can happen in a single line.
Actually drawing things on screen depends on two things:
The first is racing the beam. The display output traces across the entire screen at 60Hz, one scanline at a time. At no point does a complete image exist, instead you just make sure to apply changes so that the machine can draw what needs to be drawn just before the beam traces that part of the image. You will need to cycle count so that your program takes exactly the right time to execute every section, because you certainly won't have time for interrupts or other synchronization.
The second is using dedicated hardware, where you store the location on screen, color and memory address of a sprite, and the hardware draws it for you. There are a very limited amount of sprites available, which limits the amount of things that can happen in a single line.