How to sign a TX offline by Solana SDK? And then send it afterwards

I need to prepare in advance, for a month, a 100 of TXs signed in offline, for SOL and USDT, USDC. In the documentation - offline transactions - it's described how to sign them via the cli utilities which isn't what I need to.

How would I do this via the Solana SDK/web3, in Rust?

And how would I then send or broadcast them when I need to? For in the SDK there's a method signAndBroadcast(...), whereas in the case of offline signing, for broadcasting there'll have to be used a separate method, right?