Research - how do artists visually record sources for inspiration

Leonardo Da Vinci painted the Mona Lisa in the early 1500s. The painting was off a women called Lisa Del Giocondo, the wife of a wealthy silk merchant. It is said that he painted her with no jewelery, objects such as a book or musical instrument, clothing embellishments, lace or fur or anything that would identify her to be upper class.
I think the reason he done this was one of many, he adored the women herself and thought she had a wonderful smile so he wanted to paint her as if she were a normal person of natural beauty, he used her as a muse.

This is a painting by the artist Van Gogh, he painted it it paris 1887.
It is a painting of a bunch of bright and withering sunflowers in a vase with a yellow background. He moved to paris in 1886 - 1888 where he met some impressionists, the Impressionist's use of light and color inspired Van Gogh on updating his own palette.
Which is one his main inspirations which also inspired him to do paintings like these sunflowers, it is also a symbol of happines, which was a huge part of Van Goghs life.