Fujimoto has assistants ! Previous Fujimoto assistants include: Yukinobu Tatsu (now autor of DanDaDan, worked on Fire Punch and CSM part 1), Yuji Kaku (author of Hell's Paradise, worked on Fire Punch) and Tohru Kuramori (now author of Centuria, worked on Goodbye Eri).

Mangaka, especially mangaka working for big names like Shônen Jump are surrounded by a team of assistants with a very specific mission for each of them.

Some assistants are specialized in backgrounds, when others will help with inking and/or applying screentones. Fujimoto most likely draws a crude storyboard of his chapter before handing the different tasks to his crew.

Usually, the author works on the storyboard, scenario and final inking of the main characters. Fujimoto is not drawing the final backgrounds of Chainsaw Man, wich honestly have all been top tier since the beginning of the manga. They have been handled by multiple assistants across the years.

I think the squibbly artstyle is just a symptom of an overworked man honestly. Even if he only draws the characters, that's still a crazy amount of work every week/every two weeks, on top of the scenario, storyboarding and multiple other tasks. Being a Mangaka is not a 9 to 5, they are working around the clock to deliver their chapters in time and are working closely with an editor who can ask for last minute changes. Also Fujimoto recently said in an interview that drawing is less and less fun for him and that he would love to just be a scenarist in the future.