How to initiate PDA accounts on program

[lantianlaoli]

I need to handle an uncertain amount of PDA accounts on an instruction. so I passed them as remaining accounts. but, I need to initiate anyone if it hasn't been initiated. so I found two functions create_account and create_account_with_seeds on anchor-lang-0.30.1/src/system_program.rs, which may suit my requirements.
Question 1: what do the parameters mean on CreateAccount?

#[derive(Accounts)]
pub struct CreateAccount<'info> {
    pub from: AccountInfo<'info>,
    pub to: AccountInfo<'info>,
}

Question 2: seed: &str and pub base: AccountInfo<'info> on CreateAccountWithSeed, both of them still not made clear to me what I should correctly pass in, seed is str, but my PDA account was build by two string. the base is especially unknown what does means.

#[derive(Accounts)]
pub struct CreateAccountWithSeed<'info> {
    pub from: AccountInfo<'info>,
    pub to: AccountInfo<'info>,
    pub base: AccountInfo<'info>,
}

and

pub fn create_account_with_seed<'info>(
    ctx: CpiContext<'_, '_, '_, 'info, CreateAccountWithSeed<'info>>,
    seed: &str,
    lamports: u64,
    space: u64,
    owner: &Pubkey,

Some code use cases on github cannot solve my doubts.