Virtually anything that based on "time" rather than goods/equipment, i.e. consultancy, coaching, etc. Or if no such "skills" then do something labour intensive such as cleaning, gardening, car washing, etc where start up costs are low.
Or you start small and "boot-strap" or "shoestring" it, i.e. buy something cheap, sell it, re-invest the profits to buy more, or bigger things, sell them, re-invest the profits, rinse and repeat dozens of times as you "ratchet up" the business.
I set up a small accountancy practice (initially self only) with just my "Time" desktop and cheap free printer back in 1999, using free software (unbranded basic etc) that came free with the computer - VERY basic spreadsheet and VERY basic word processor. I'd previously bought the computer for home use so no additional cost. Advertised by putting cards in local corner shop windows. Got a handful of clients, used the fees I charged to advertise in the local newspapers, then used the fees from the nex tranche of clients to buy proper software, and so on, until I rented my own office, bought a decent laptop, computer, scanner, printer, shredder, etc.
One of my clients was a classic shoe-stringer. She had a little hobby and started selling surplus stuff on ebay back in the mid 00's, just odds and sods really, where she'd bought a pack of 10 of something but only needed 2 or 3, so sold the rest. Rinse and repeat several times. Then she started buying stuff purposely to sell, rinse and repeat. Then she got a proper website and ditched the ebay account. Went from strength to strength, eventually £2million turnover and she sold out for around half a million after trading for a decade or so. Again, started basically from nothing, just spending a few pounds here and there on her hobby and then it grew organically.
Another client sold wet fish from the back of a clapped out van. Spent all he had every day at the wet fish market first thing in the morning and went out and sold it - used the cash to buy more stock the next day, rinse and repeat! After a decade he had a food processing factory selling sandwich fillings to national chains, likewise sold out for a few million. Just by "ratcheting up" his business and going into different products/suppliers based on networking contacts made as his business grew and changed into different markets.
The ones who had shed loads of cash to invest , i.e. from inheritances or early retirement lump sums tend to "waste" it and my experience is that they're more likely to fail and lose it. Usually because they go all out with expensive offices/premises, unnecessary equipment, leases, expensive cars, etc. Basically money burning a hole in their pocket and they just spend it because they can, wrongly thinking that "image" makes a successful business (it doesn't!).
If you've no money, you're more likely to be careful about how you spend it, what you can do without, make things out of nothing, "make do and mend". No it's not easy having to work like that, but at least you're not building up debt/loans/obligations etc that you struggle to repay.