It depends on your reading level and familiarity with the genre. If you're used to reading light novels, it may not be difficult. The language is usually accessible and the story - telling is engaging.
For some readers, it's not difficult at all. The plot of Solo Leveling is engaging and the writing style is straightforward. However, if you're new to web novels or not into the fantasy and action genre, it might take some time to get used to the pacing and the world - building concepts.
There are also some official websites of the publisher or the author that might provide the option to read the 'Solo Leveling' novel. However, make sure to access only legal and authorized sources to support the creators and avoid any copyright issues.
You can look for authorized e-book platforms or apps that offer the Solo Leveling light novel and check if they have an iOS version compatible with your iPhone.
They fill in the gaps. There are many parts in the main story that left us wondering about certain characters or events. The side stories answer those questions, making the overall narrative more complete.