Set up an experiment. Find the result.
Move the experiment somewhere else and run it again. You will get the same result.
To do this, you must move all the important parts of the experiment. For example, if you drop a rock on earth, it falls. If you move the experiment out into space, it just floats. To do it right, you would have to move the earth too.
The results would not be exactly the same because gravity from the moon and the sun have a small effect. So you really need to move them too. And if you really get precise, everything in the universe has a small effect. So you need to move the whole universe. And if you do that, how do you know you have moved anything? It gets confusing.
But if you just move the earth and the thing you drop, you would find that nothing about space itself is different in the two places. The difficulty is all in setting up two identical experiments.
If Alice is in a rocket in space with the engine pushing her, she is in an accelerated frame of reference. If she drops a rock, the rock falls toward the back of the rocket.
Alice is using a frame of reference where things are motionless if they keep up with the rocket. She sees herself as motionless and the rock as accelerated.
An inertial frame of reference is one with no forces on it. If Bob is floating in space far from earth, a dropped rock does not move. In this frame, $F=ma$. If he ties a string to the rock and pulls on it, the rock accelerates.
Bob can make sense out of the rocket. He sees Alice and the rock accelerating together. When Alice drops the rock, it is left behind. Alice accelerates ahead of it. You can see how it looks to her like the rock is accelerating toward the back.
To Bob, space is isotropic. Bob can face any direction and get the same result from his experiment. Space has no special direction.
Alice does see a special direction. She has to exert a force toward the front of the rocket to hold the rock motionless. If she stops and lets $F=0$, the rock accelerates toward the back.
Alice finds that space is homogeneous. She gets the same result if she moves to a different place in the rocket and drops the rock. This sounds like splitting hairs. But it is sometimes useful to distinguish between isotropic and homogeneous.
Alice wants to do physics too. She wants to use a law like $F=ma$. But $F=ma$ applies in inertial frames, and Alice is ignoring a force that pushes her and everything she sees forward. To make it work, she has to play a trick. She says a fictitious force is pushing everything back. She adds this force to the force she exerts on the rock. They cancel and the total force is $0$. Now $F=ma$ works.
One of Einstein's great insights was that gravity is a fictitious force just like a rocket engine. This is called the equivalence principle. This is the basis of General Relativity, which is a theory of gravity.
Bob cannot do any experiment that tells him whether his rocket is floating in space or falling off a cliff on Earth.
Alice cannot do any experiment that tells her whether her rocket engine is pushing her forward or if the rocket is sitting on earth with the engine off.
This is a bit confusing, because Bob or Alice can just look out a window. But that doesn't count. The idea is that for the space inside the room, the laws of physics are the same whether the acceleration comes from gravity or a rocket engine.
So the surface of the Earth is an accelerated frame of reference.
Space near the earth is not homogeneous or isotropic.
If you do not move very far, space is almost the same. But if you go up, the force of gravity gets weaker. If you go to the other side of the world, the special direction changes.
General relativity is not obvious. It might not have been the simplest example of how space can be inhomogeneous in a non-inertial frame of reference.
It took 300 years to get from the discovery of Newton's laws to the discovery of General relativity. For those 300 years, people have been treating space around earth as homogeneous and isotropic. Gravity has been treated as a real force. This all works just fine in ordinary circumstances. You should think this way too.
General relativity is useful because it explains how mass causes gravity. It explains tiny differences between predictions of Newton's laws and experimental results. Starlight is deflected a tiny fraction of a degree as it passes near the surface of the sun. The orbit of mercury is almost but not quite an ellipse. Time runs very slightly slower on earth than in orbit above. It explains effects produced by extremely strong gravity, such as black holes and gravitational waves.
