It's called margin and as such you can withdraw cash with most online trading accounts.
So if account is worth $3M you can withdraw 50% or $1.5M for sure or up to 70% ( or over $2M) with some stocks and mutual funds eligible for lower 30% margins.
Check your margin requirements and eligibility. Interest rates are usually prime plus 1% and sometimes lower with accounts over $1M.
If stocks and mutual funds are held with a traditional ( aka expensive ) broker move all stocks and MFs in kind to a new online account with margins.