Most successful business owners will have a history of other businesses behind them, some successful, some not. Very few jump straight in to businesses requiring high amounts of capital.
Plenty of people start very small with small scale businesses that require very little upfront cash. They reinvest the profits to make their "small" business bigger and then ultimately sell it and use the savings/proceeds to ratchet up to a bigger business, and so on. Lots of businesses don't have high "barriers to entry" whether cash or qualifications or experience, etc. Another good thing about starting small and having a series of small businesses as you grow is the experience you gain, i.e. dealing with and making contacts, with suppliers, customers, banks, insurers, accountants, solicitors, etc, all of whom you'll need when you come to buy your "big" business.
An alternative route is working and saving as much as you can over several years.
Nothing at all wrong with starting small/simple and growing/expanding as the years pass by until you get to where you want to be.
I'm an accountant and I've had 2 "self made millionaire" clients over the years. First one was a guy who had a workshop in his garage - he spent years developing a specialist piece of equipment (electronic connector for a specific industry), getting patents for it, and finally launching it. After 10 years of manufacture/production, he sold the company for £20million. All from a garage workshop he cobbled together and a few years of working on the designs/prototypes etc.
The other client was a husband/wife. Wife was a SAHM with 2 young kids. Husband was a shop manager. So, pretty normal. Husband got diagnosed with a debilitating disease so had to give up work. Wife had to do something quick to pay the bills. She had a hobby and had been frustrated with difficulty getting materials/equipment for her hobby. She decided to try to source these, and sell them on Ebay, as she thought there may be others with same problem. She bought £250 of stock (all they could afford). It sold very well, so she bought more stock with the proceeds, and so it went on. Eventually she ended up with a warehouse and 20+ staff, and then sold the business for £5million.
Both started from basically nothing but what they already had and ratcheted up their businesses from nothing by reinvesting the profits.