Here is a question worth being able to answer, because interviewers ask it: why do Playwright tests not interfere with each other, even running in parallel?
The answer is the browser context, and understanding it explains several behaviours that otherwise look arbitrary.
Three layers, from big to small.
The browser is the running browser process. Starting one is slow, so Playwright starts few of them.
A context is an isolated session inside that browser. Its own cookies, its own localStorage, its own cache. Think of it as a fresh incognito window. Creating one is fast, a few milliseconds.