Every ROS 2 topic can be inspected from the command line with ros2 topic echo — but a LaserScan message is a flat array of 360 floating point numbers, and a map is a giant list of 0–100 integers. RViz2 subscribes to those same messages and draws them instead of printing them.
Technically correct. Not something you can glance at and understand.
The same 360 numbers, drawn as points around the sensor. Obstacles are immediately obvious.
RViz2 works through displays — small plugins, each built to understand one message type. Every card below runs a small live simulated preview right in the page — the last two are hands-on: the controls actually drive the visualization, the way a real display responds to live data.