[xiphias2]

> Technically, BTC “transactions” can happen "on paper", the keys can be physically handed off to each other, etc (there are plenty of possible answers here that are all fairly vague and hand-wavey). There are many problems with this that make it wildly impractical though (see replies to this comment).

For large transactions using microSD cards is the recommended way actually. There are multiple hardware wallets that support partially signed bitcoin transactions (PSBT), and using SD cards both make it easier to verify the communication between the signing device and the Bitcoin network, and make it easier to sign the transactions using multisig at multiple different locations before publishing them on the network.