Hi RosiePosie
I will take you desired end result is to be able to combine image components that are either already illustrations or that you want to convert existing real life images to illustrations and combine them in a document.
@JeffK gave a good starting point for converting real life images into watercolors
So there are two topics I see. How to get images that are illustrations and second how to combine them
If you are happy with using illustrations that already exist, there are many online sites that provide such images and a number that also have free images too. This could save a lot of time if they hae what you want. Here is a link to one such site:
https://pixabay.com/
That site initially will show images that you need to pay for followed by many more images that are free. You just use the filter to have it show you illustrations only to give you a choice. Here is an example cut and paste from that site and notice the checkerboard is indicating that it comes in a format with transparency surrounding the image for easy adding into your overall product:
View attachment 115519
To add these elements into your image you can use the File > Place Embedded and such image will be added as its own Layer.
I will stop here as I am not sure this is the path in which you are interested or not.
Hope this helps with a starting point and just add more comments to help steer the feedback you are getting.
John Wheeler