Usually when a novel is transformed into an animated movie or manga the author is well known for their work but many companies scout out popular novels or ones that have immense potential. They ask the novelist if they'd like their novel to be turned into a manga or movie and if the novelist agrees then a manga or movie is created. Its good for the author to get some good advertisement and they can make more money then they did before.
It's that simple really. Publishing companies scout out popular novels that could be turned into manga and if the novelist agrees then BAM manga. Sometimes the novel doesn't have to be popular but that's not something you see everyday because it is a risk to take with a unpopular novel. For example, Sword Art Online use to be a light novel but was turned into a manga because it was already so popular in Japan and then an anime came out for it. However it is hard because if you want it to be turned into an anime you have to be popular in Japan first. Another example is Howl's Moving Castle. Howl's Moving Castle was originally a British novel. Studio Ghibli took interest in the novel and got permission to make an animated movie based on it. Some novels/ light novels already have some pictures in it (like a picture book except it doesn't have many pics) drawn in the style of a manga but they can be given the opportunity to make it a full fledged manga.
I hope this answers your questions but if you have any more don't be afraid to ask. I try my best to answer them. Tbh it'd be better to research this yourself because i may miss a few details. ╮(￣▽￣)╭
Thank you so much!
I never knew that SAO and Howl's moving castle were adapted from novels!
Yeah if you do a bit of research a lot of famous works come from novels
Can someone please explain to me how a novel got turned into a manga?
The person who wrote the novel - is he/she just a person living his/her daily life, and simply decided to spend a portion of their time to write a novel online?
How does the novel gets turned into a manga? Does a publishing company contact them to use their story? Is the mangaka someone that's already licensed as one? Or just someone else out there in the world who decided to draw manga ?
Much appreciated