Mag tape encodes the data digitally in magnetic domains. Typically 8 bits in parallel across the tape with a 9th parity bit on older tapes. Later they went to burst error codes which were more reliable and efficient.

We can't 'read' the Apollo tapes simply because we won't spend the money. The technology is easily reverse engineered even without documentation or knowing the particular encoding scheme. If it had Al Quaeda info on it the CIA would have it recovered quickly. If a future civ found either CDs or tapes where that data was somewhat well preserved it could readily recover the data but likely it would ONLY do so if the effort & expense was worth it. We've simply decided not to spend the money to read the old Apollo tapes. Spending the money analyizing current mission data is the priority.