You can't rollover non -retirement money into a retirement account.
You can do a rollover from the 401k into a private retirement account called an IRA. The reason you might do this is that 401(k) often have limited investment choices (because the employer has a responsibility to make sure you don't go too crazy with your money) while you can invest in almost anything in an IRA. The drawback is that large employers are often able to negotiate lower fees for their investors, and you may pay higher fees on the same investment in an IRA. A rollover from a 401k to an IRA is not taxable or reportable as long as it is direct (trustee to trustee). Contact both plans ahead of time to set it up.
If you take withdrawals from a 401(k), you will pay regular income tax. You must start taking withdrawals in the year you turn 72-1/2, this is called an RMD or required minimum distribution. You must withdraw at least that much, although you certainly can withdraw more. The RMD amount is calculated from the account balance and your life expectancy. It is not a special transaction, just the amount you must withdraw and pay taxes on. For example, if your RMD is $5000, you could satisfy that by withdrawing $5000 on December 26, but if you had been withdrawing $500 per month for living expenses, you would already have met your RMD for the year and no special withdrawal would be needed.
Stocks and other investments are taxable every year, not just when you withdraw them, however, you don't pay tax on your entire portfolio. You pay taxes each year on interest and some of the dividends earned during the year. But you only pay tax on the principal when you sell an investment, and then you only pay tax on the gains -- the increase in price. For example, suppose you buy stock in ABC company in 2015 for $100/share. The stock pays $1/share in dividends every year, that is taxable when you receive it. You sell the stock in 2022 for $120/share. The $20/share increase in price is a long-term capital gain, and you pay tax on that, but not the entire $120 selling price. All your investment income should be reported to you on a 1099-B statement issued by your broker each year that you include on your tax return. If you have investments not held by a broker, you have to track your own paperwork.
