For example, what happens if I buy 10k in stock with personal income,
and the stock goes down 5k, and I withdraw 10K. Will the extra 5K I
withdraw be taxed or not?
You can't withdraw money you don't have, if the value your 10k investment is cut in half the most you could withdraw is 5k.
In a Roth IRA you contribute post-tax dollars and can therefore withdraw your contributions before retirement with no tax burden and no penalty, unlike a 401k or traditional IRA where withdrawing early could result in a penalty on top of the taxes owed from taking the withdrawal.
IRA's are tax-advantaged and should be prioritized, most people prioritize maximizing annual IRA contributions 2nd only to getting their full employer match on 401k (if applicable).
The main downside of an IRA is the low annual contribution limit which makes it impractical to use as your sole instrument for retirement saving/investing.