The money that you have been contributing to your 401(k) plans has not been taxed yet. You have deferred the taxes that were due for years but at some point, the government wants its share. And that tax applies to all the money that is in the plan but not all at once.
The earliest you can start taking money out of your plan is when you turn 59 1/2. Thank the government for adding the 1/2 year to make things unnecessarily complicated but once you turn 59 1/2, you can take money out of your pre-tax accounts like 401(k)s and IRAs penalty free. If you take money out before that, you pay a 10% early withdrawal penalty. Taxes are still due on the money you take out.
You can take as much of your money out from your plan in a given year but whatever you take out, it is counted as income on which you pay the required tax. You will receive a 1099-R that will state the amount you took out that you will then file with your tax return.
You can choose not to take the money out at 59 1/2 years of age and let it continue to grow tax deferred. But the government isn’t going to allow you to defer taxes forever. You must start taking money out of your pre-tax accounts when you turn 73 or at the latest by April 1st of the year after you turn 73.
I would not wait till April 1st of next year once I reach age 73 because then I would have to take two RMDs that year (the first by April 1st for the previous year and the second by December 31st for that year). That would be bad, taxwise.