The Atari 400 and 800 are interesting to me because they had a different video architecture from most of the other computers of its generation. Most systems of the day had a simple frame buffer and/or “tile” system where the tiles are defined by a character ROM so you could draw an A to the screen by writing a 65 to the right memory system. Some systems (like the Apple ][) could work in a split screen mode where part of the screen was bitmapped and another part was text but this was an exception instead of the rule.
The Atari machines on the other hand had two chips that worked together to prepare the display data which meant that you could choose a different video mode for each line which gave developers a lot of flexibility and reduces the display RAM requirements for a machine that launched with just 8k of RAM. (For instance if a horizontal line was all one color you did not need that frame buffer line; if you wanted to do horizontal or vertical scrolling that could be configured by setting the offset of where scanning starts)