IIRC it was torches there seem to be several archaeological finds

This is one of the problems with what the Celts did: they didn't write. And they used a lot of biological material (like leather, wood) which is all gone by now. AFAIK the celts were already "gone" (assimilated / diversified into local tribes) when the Romans (which were writers) arrived. So we don't even have roman writing about the celtic customs.

For example, there is are excavations around https://de.wikipedia.org/wiki/Keltenwelt_am_Glauberg and AFAIK it's still not clear if that area is mostly a castle, or if it was mostly some cultic thing -> nothing written, not even in clay or chiseled into stone.

Similarly, in this article https://de.wikipedia.org/wiki/Hallstatt_(Arch%C3%A4ologie) that describes the salt mining in Hallstadt since the bronze age there is only so much that they know.

So, did they use torches? They have decayed over this time.

We know however that they had some kind of meat industry at Hallstein. They had plenty of salt, and so they used that to make meat durable. Animal skulls survived the times ...

I would expect oil lamps.