During a IBD : Initial Block Download by a full node, from where is the blockchain downloaded?
Yes, they download blocks from other nodes they connect to.
In Bitcoin Core you have the choice between being a pruned node or not. In both cases all blocks are downloaded and fully verified, but in pruned mode old blocks are deleted after a while after being verified. This has a number of implications, but one of them is that a pruned node does not help new nodes perform IBD.