Is the UTXO index in RAM or is it a file system object?

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The blockchain contains a lot of redundant information and a full node will typically build up an index from the blockchain for efficient transaction verification and generation. I’m wondering if that index exists in RAM or in an OS file system? On the one hand, swap memory should be a last resort and doesn’t scale, on the other hand, at 4GB it probably fits into RAM depending on hardware and doesn’t need to be persistent. What does Bitcoin Core implement? Or is it a file system object with RAM caching?