Questions about the types located in the file `/source/basic.d.ts`

[doox911]

Source: https://github.com/sindresorhus/type-fest/blob/main/source/basic.d.ts

1. Why do we specify ?: when the property might not exist due to | undefined
2. Please, provide a practical use case types: JsonValue, JsonPrimitive, JsonObject, JsonArray, so how can one perform type conversion using as after JSON.parse?