How do you go about starting an E-commerce business like Shopee, Lazada, or even something like Shopback?

If it were my business, the best solution I'd consider would be to go to the specialists and experts who work with Magento. I already have experience with amasty specialists, who in addition to a huge number of top plugins and extensions also provide magento website maintenance services. It is literally a whole complex of different services, which includes the integration of payment solutions and full support for your online store.

Traditionally speaking, an online marketplace is one of the least capital intensive business model (usually asset-free or asset-light at the beginning). However, the downside is that it has a low barrier of entry.

To figure out how much money you need to start an online marketplace, you’ll first need to identify and understand the problems you’re trying to solve, what makes your value propositions unique.

From there, you’ll be able to define your go-to-market strategy by gauging how strong your value propositions are (marketing, sales, management, execution, product or tech superiority).

When it comes to your go-to-market strategy, are you planning to grow at all cost? This means acquiring suppliers (merchants) and demand (customers) at a loss-making rate to carve out significant market share quickly. If that's the case, you're going to need a whole lot more money to survive the bloodshed.
(E.g. None of the companies you've quoted is in the green. They’re still burning millions of cash year on year)

Or do you plan to focus and invest your resources in building a fantastic product (the marketplace platform) while using tech and product superiority as the main differentiator?

More often than not, having a fantastic product (your marketplace platform and its network effect) increases the chances of winning over the market share in the long run while maintaining a decent unit of economics.

Quoting Mark Zuckerberg’s advice to the Airbnb founders when Wimdu (+90 mil funded, dominated the market share in Europe) came knocking for an acquisition, “Don’t buy. Whoever has the best product will win.”
Airbnb is now a profitable multi-million company, while Wimdu has ceased its operations in 2018.

In today’s context, the $50k that you've spent years saving can be expended by a company in a matter of minutes.

Hope this helps!