Well the solar return gives an overall view of the year ahead. Each planetary return is cast the moment it enters the same sign and degree of the position of that planet in your natal chart. This will give you a look into your future in regards to the affairs ruled by that specific planet. Venus, for example rules love, creativity, beauty and money. So these are the things that will be at play.
To illustrate more clearly on how a different planetary return is cast, it’s like this. Perhaps your Venus is 2 degrees Aquarius. Well every time transiting Venus hits 2 degrees Aquarius, a Venus return is drawn. (And actually it’s more precise then just 2 degrees, but gives you an idea)
A solar return is cast once a year, more or less exactly. Give or take a day or two. Other planetary returns aren’t as exact. And not always yearly. The outer planets take longer. The inner planets can be shorter. There’s also retrogrades that can slow things down.
The chart that is drawn looks identical to a natal chart and is read similarly. You’re just seeing the upcoming forecast of affairs in your life ruled by that planet. It’s good info to know.