I’m not the most qualified person to answer the technical questions, but Lightning is a scaling layer on bitcoin, so it relies on a fully-synced bitcoin node to function. There is more than one implementation, so node operators can choose which one they prefer. For example, if you want to run LND, you would install that software on the same system as your bitcoin node so it can refer to a local copy of the blockchain to perform channel actions.

Tagging a few people here who can probably give you better answers.

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