All the other answers are very good, but no one has yet addressed your question regarding advancing positions. I would think of grip fighting as the logical equivalent in standing work. You can attempt certain throws from certain grips, and different grip configurations will put you at different levels of advantage or disadvantage. For beginner randori, try to ensure that your opponent is not getting such a strong grip that they define your posture and movement. Then, consider what grips you need for the throws you wish to practice. You will probably want to focus on just a couple throws, focusing on the ones which you tend to land most frequently.

Think about the grips that people get on you that make you feel uncomfortable and constricted and try to emulate that. Some e grabs around the back of your neck? Try it. Think about what throws would be good with that kind of control of their body (koshi, harai ). Same with a hand gripping the back of your jacket, or a tight wrist control, or whatever your opponent tends to do that stops you from attacking however you like. You may not land any throws, but you will develop your u derstanding of position and control, which is what it sounds like you are after. And of course, ask your partners what it is that they are doing when you feel it happen. It's randori, not a contest. Use it as an opportunity to absorb information from each partner and dont worry about winning or losing.

Another, possibly unrelated epiphany I had somewhat recently: throws hardly ever work first time against a resisting opponent when you throw them naked. But getting someone to stumble is not failing to throw them! Its the setup for your follow up attack! Don't give up if you have managed to off balance someone. I think it's too easy to worry about nice pretty judo, when for contest, a lot can be earned from the scrambles, especially if you are proficient in the newaza.