Juice and syrup are two different things, though they are liquids. Both words are not related in any way, meaning they can't be used interchangeably. Juice simply refers to a liquid from a plant, especially fruit. This liquid is usually extracted from the fruit through different kinds of processes.
For instance, you can easily extract the juice from orange by squeezing it. On the other hand, syrups are thicker liquids compared to juices and are even more viscous compared to the latter. The extracted liquid from fruits comes naturally, whereas syrups are artificially prepared by adding dissolved sugar to other liquids.
However, in some cases, syrups can be prepared by adding two or more juices together. The extracted liquid from fruits is naturally sweet. In contrast, you will need to dissolve sugar in a mixture when making syrup. However, there are some syrups that are made without the regular sugar, instead of substitutes like sorbitol and glycerol are being used.