Not much.
Like Magna Carta, these things tend to be only of historical interest because their significance is bound to the conditions of their times. Quia emptores is for all practical purposes superseded by much more modern arrangements about land ownership.
In discussing this, it is also worth pondering what you consider “ownership” to mean. When you “own” a thing such as a car, you tend to think that what you own is the car. Sounds obvious. But a more sophisticated view is that what you really own is a bundle of rights that you can expect to be enforced against the whole world, such as the right not to have it taken away from you by someone who is merely stronger than you, the right to drive it wherever you reasonable want to go, the right to sell it to someone else, and so on. And these rights become increasingly complex and contingent - the right to own the car is subject to the obligation to pay taxes, and so on.
Thinking about landholding is similar. When a feudal sublord held land from the King, what he “held” was a bundle of entitlements, which were subject to obligations. They were very different from what we would now think of as the entitlements and obligations attaching to land ownership, but the principle is the same. And Quia emptores was part of the historical process of changing the typical contents of the bundles in the interests of economic efficiency and as a reflection of changing social conditions.
Thus, what has happened in between the time of Quia emptores and now is that the principles of land holding have radically changed. What has also changed is that at the bottom of feudalism lies slavery. This is, in essence, inescapable if you have a system of compulsory obligations passing down the line.
Let’s suppose you think of the King as owning all land, and his ownership is unalienable because he got it from God. That means that what the King feudally “grants” to the Earl of Chinless Wonder is not “ownership” of land in the way we would presently think of it, but the right to profit from it, subject to a complicated series of obligations.
Because of the primitive economic principles in play back in the day, the Earl couldn’t as a matter of practicality just sell it and live on easy street, because there were no banks or share markets in the modern sense where you could safely put your money and earn interest. In any event the King might be pissed off, and not let you or the prospective purchaser play together like that, because after all it was his land, not yours. In a very real sense, land was wealth (in the sense of being “capital”) and money was just for paying expenses. The idea that money could itself be capital was some distance away. Bear in mind also that in the feudal system, much of the land was unalienable because it was entailed - it had to be left to children according to the laws of primogeniture. But that is a different problem.
Part of the entitlement of a grant to the Earl of Chinless Wonder was that the Earl of Chinless Wonder could profit by sub-infeuding to the Baron of Lesser Bottomtooth who would then have obligations to the Earl of Chinless Wonder that were cognate with those that the Earl owed the King. And so on - in principle, it was turtles all the way down.
Up to a point, this can be economically efficient - the Baron may be a more efficient land manager than the Earl, for example - but very rapidly it becomes apparent that too many people are trying to live parasitically off the finite amount of land available. Moreover, as subinfeudations increase, the idiosyncratic obligations negotiated piecemeal throughout the country become unmanageable.
With the rise of a reliable medium of exchange, urbanised life and so on, the idea that one might actually want to sell one’s land became sensible, even desirable. Over time, landholding became standardised as an estate “in fee simple”, something that we would now recognise as ownership, even though it pays lip service to the idea that the Sovereign owns all the land. Of course there are exceptions in the UK derivative of history, such as the practice of purchasing 99 year leases from the Duke of Westminster, for example, but the benefits of standardised and economically efficient relationships between landholding subject and King are obvious.
What also became obvious was that the feudal process of obligation - the baron had to supply troops at the Earl’s call, and the Earl had to provide troops at the King’s call, and similar obligations with respect to servitude - was inefficient. It became clear over time that it was more sensible to simply pay money instead and let the King organise his army or his workers centrally. The process of obligation to provide workers of various sorts obscured the fact that there was a fluctuating market for workers - when the plague struck, the cost of labour went through the roof, for example. Feudal obligations of servitude thus became difficult to manage as the cost of the labour provided varied. Again, the language to think in these terms did not exist, but the rules of economics always have.
Despite the lack of a language of economics, quasi-Darwinian processes made it inevitable that the inefficiencies of sub-infeudation had to pass into history. That way of thinking about land ownership had, in essence, simply ceased to be relevant.
The modern process of leasing land has some superficial similarities to the old feudal system in the sense that in principle, the process of subleasing is limitless, but that is not the same thing is infeudation. Different bundles of rights. For example, a lease is for a finite time. We have learned about the perils of perpetuities. And the sublessor has bargaining power in a free society that was radically distorted in a feudal one, so that a modern sublessor can readily see that he is paying too much for his lease if his earnings have to support layers of freeloaders above him, and go somewhere else.
In summary, Quia emptores is an historically important step in the unpacking of primitive feudalism, but no-one cites it today in any actual legal setting except for historical reference.
As to whether a person can in the present day rent land for labour, sure, why not? People are free to contract. If Beauregard Billionaire lets Harry Hobo live in his shed on the basis that Harry paints Beau’s porch, why would the law intervene? There may be individual restrictions on the process in various places to prevent abuse or exploitation, but the difference between this and feudalism is that Harry is free to leave without being hunted down by dogs - serfdom is not at the bottom of the pile. If he doesn’t do the work, he can be sued. If he does and is kicked out of the shed anyway, he can sue. The bargaining power may be lop-sided (it always is) but it is not non-existent.