greenrivet, first WELCOME. You will find people here friendly and willing to do their best to help you.
Let me start with the observable Universe. Think of standing in the centre of the town or village where you live. It us the middle of the night, and very dark. For some reason, there is only one street light working. You stand still and gradually piece together your surroundings. Close by are some shops. Further away you can just see a building which you recognise as the library or town hall. Whilst you can reach the nearby shops with a few paces and within a few seconds, the distant building is at the very limit of the street light, and it might take you 5 minutes to walk there.
Now imagine that the street light goes out, and you look up at a starry sky. Some things up there are quite close, like the Moon. Perhaps you can make out Jupiter, which looks like a bright star, but through a telescope it has a disc and maybe markings. The Moon corresponds to the nearby shops. Light takes only 1.282 seconds to cover the distance. To get to Jupiter, light takes about 43 minutes. For the Sun, light takes 8 minutes to reach us.
Coming back to your town, you can see only a few hundred yards, but you know that there are several towns within 25 miles of you. You cannot see them, but they appear on maps and you know roughly where they are relative to where you are. In this example, your observable Universe on the ground is pretty small. It is only what you can see. The other towns, and overseas countries are on your maps, but you cannot see them - they are not part of your observable Universe.
When you look up into the sky, there is some similarity. Some objects, like the Moon, and Jupiter, you can see without a telescope. They are part of your observable Universe. Now let us suppose you buy a small telescope. Suddenly, your observable Universe has expanded greatly. Let us imagine that it is daytime, and you are looking at your town again. But this time you are looking through a pin hole in a sheet of cardboard. Your view is a bit limited. By acquiring a telescope you have made the difference between looking through a pinhole and looking through a metre diameter hole. It is not exactly the same, but there is some similarity. The telescope gives you a bigger 'eye' to collect more light and lets you see fainter objects that you can see with the naked eye.
Now, whilst you have just bought a small telescope, you have found out that you can see fainter objects than you could before. You previously noted a small group of stars canned the Pleiades, or the 'Seven Sisters'. Without the telescope you could make out six or seven stars in the group. Now, with your telescope, you can make out many more. Having seen how much a small telescope can help, you can imagine how much more astronomers can see with their very large telescopes. Now, astronomers can 'see' with other signals, such as infrared or X-rays, and these observations can provide much more information still.
Just as you knew about other towns, when you were standing in your town centre, you now know what a vast amount of information astronomers have been able to collect. This is the observable Universe of science today. But, still, there are other things we have not discovered. Standing in your town centre, you knew of some nearby towns - even from an Atlas, you knew of the existence of more distant places. They might have been just a dot on the map, but buying a local street plan could add detail to your personal observable Universe.
Here is a short story. The boss of the Hubble telescope had some discretion about where it was pointed. He started a program of looking at an empty part of the sky. The longer the telescope was pointed in that direction, the more time there was to collect light, and after quite a long time, the images he was collecting began to show details which had previously been invisible. The observable Universe had been extended.
So, you see, the observable Universe is just that. It is what can be observed at a given time, and it is subject to change as more observations are made. I hope this has helped with your question about the observable Universe, because then we can start expanding answers to your other questions.
Let us know how you are getting on, and if we can fill in any areas where we have been unclear.
Hope to hear from you soon.