schmuli

• target tells TS what type of JS to generate, for es5 for conversion is required (for example let/const -> var, class -> function + prototype, etc.), whereas es6 can just leave the original code as is (after removing type information).
• types tells TS about what types exists and what their API is. When using lib you are asking TS to add known core types that it already has definitions for, without having to add those yourself (for example, @types/core-js). When you import you adding more types that TS knows about. Additionally, you can use the types configuration property to add global types.
• polyfills are required at runtime to ensure that older browsers can use the constructs (such as Promise) that exist natively in modern browsers. At this point, TS is not involved at all.