I think your question might be coming from a misunderstanding of how corporate structures work - specifically, that a corporation is a legal entity (sort of like a person) that can have its own assets and debts. To make it clear, let's look at your example.
We have two founders, Albert and Brian, and they start a corporation called CorpTech. When they start the company, it has no assets - just like you would if you owned nothing and had no bank account. In order to do anything, CorpTech is going to need some money. So Albert and Brian give it some. They can give it as much as they want - they can give it property if they want, too. Usually, people don't just put money into a corporation without some sort of agreement in place, though. In most cases, the agreement says something like "Each member will own a fraction of the company that is in proportion to this initial investment." The way that is done varies depending on the type of corporation, but in general, if Albert ends up owning 75% and Brian ends up owning 25%, then they probably valued their contributions at 75% and 25% of the total value.
These contributions don't have to be money or property, though. They could just be general "know-how," or "connections," or "an expectation that they will do some work." The important thing is that they agree on the value of these contributions and assign ownership of the company according to that agreement. If they don't have an agreement, then the laws of the state that the company is registered in will say how the ownership is assigned.
Now, what "ownership" means can be different depending on the context. When it comes to decision-making, you could "own" one percentage of the company in terms of votes, but when it comes to shares of future profits, you could own a different amount. This is why you can have voting and non-voting versions of a company's stock, for example.
So this is a critical point - the ownership of a company is independent of the individual contributions to the company. The next part of your question is related to this: what happens when CorpTech sees an opportunity to make an investment? If it has enough cash on hand (because of the initial investment, or through financing, or reinvested profits), then the decision to make the investment is made according to Albert and Brian's ownership agreement, and they spend it. The money doesn't belong to them individually anymore, it belongs to CorpTech, and so CorpTech is spending it. They are just making the decision for CorpTech to spend it. This is why people say the owners are not financially liable beyond their initial investment. If the deal is bad, and they lose the money, the most they can lose is what they initially put in.
On the other hand, if CorpTech doesn't have the money, then they have to figure out a way to get it. They might decide to each put in an amount in proportion to their ownership, so that their stake doesn't change. Or, Albert might agree to finance the deal 100% in exchange for a larger share of ownership. Or, he could agree to fund all of it without a larger stake, because Brian is the one who set the deal up. Or, they might take out a loan, and not need to invest any new money. Or, they might find an investor who agrees to put in the needed money in exchange for a a 51% share, in which case Albert and Brian will have to figure out how to split the remaining 49% if they agree to the deal.
The details of how all of this would work depend on the structure (LLC, LLP, C-corp, S-corp, etc), but in general, the idea is that the company has assets and debts, and the owners can have voting rights, equity rights, and rights to future profits in any type of split that they want, regardless of what the companies assets and debts are, or what their initial investment was.