[mister_tee]

My understanding, anyone is welcome to correct: the 51% attack doesn't apply, but that's because there is a central party with full control from the start.

CBDCs are digital currencies but do not have to be cryptocurrencies with distributed consensus, blockchain, proof of work, etc.

Instead they are likely to be completely centralized; at an (absurd) extreme a global Excel sheet with edit access enabled for the Treasury Secretary and Fed Chair.

[credit_guy]

But then how is it different from regular currencies?

Take the US dollar. It's mostly numbers in databases. Physical cash (dollar bills and coins) is a tiny fraction of the total amount of money in circulation. By now all major currencies are "digital", and most likely all non-major currencies too.