Hello I am new to Photoshop and need to create posters like this one attached.

Probably created from scratch but nobody can't say by whom if there isn't data attached. You do not need to paint or capture images in order to create something in Photoshop including posters. Usually illustrators are doing their job, designers their. They can work for the same company or to be independent contractors. Designer can purchase or download illustration from the internet too. Many variations there. If you can not find illustration you need then try to contact some illustrator to draw/paint it for you. Low budget? Lesson learned for the next job.

Hi @NewFolk there is no way telling if this was created custom by an artist for the poster, but there are a bevy of resources available including Adobe Stock that have painted images for purchase/use.