Any enters abilities of the copied creature will trigger when the token enters. Any "as [this creature] enters" or "[this creature] enters with" abilities of the copied permanent will also work.

Each spell with paradigm has the Lesson subtype. The Lesson subtype appears on instants and sorceries and has no inherent rules meaning. Other effects (such as the learn ability from the original Strixhaven release) may refer to it.

If the copied permanent is a token, the token that's created copies the original characteristics of that token as stated by the effect that created the token.

If the copied permanent is copying something else, then the token enters as whatever that permanent copied.

Once a spell with paradigm has resolved, it doesn't matter what happens to the card in exile. Even if the card leaves exile, the delayed triggered ability will still trigger during each of your first main phases, and the copy will still be created.

Paradigm creates a delayed triggered ability that triggers at the beginning of each of your first main phases for the rest of the game. When that ability resolves and the copy is created, you can choose not to cast the copy. In that case, the copy will cease to exist the next time state-based actions are checked. The delayed triggered ability will still trigger again at the beginning of your first main phase on your next turn regardless.

The token created by Echocasting Symposium copies exactly what was printed on the original creature and nothing else (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on. If it is a Vehicle, it is not crewed.