If doesn't have any event, can we go inside for visiting?

The lobby and the gardens were designed to be a public spaces. There are tours available each day (guided, self- guided). Entry inside the concert hall itself may depend on availability.

Yes, you can go Up stairs and see the beau of the roof made it of wood. Also there is a garden and small amphitheatre and you can take great pics

I don't think so.

Yes

A week ago I went and at the entrance they asked me for the entrance ticket (which can be purchased in the booths below or online) but I think it was because they had an event.

No it's only for parking they have security

I don't know if we can go