Here's an anecdote from the other side: One of the companies I worked at had a posting open for 9 months. We've regularly re-posted the job. The manager reviewed the applications, rejected like 90% of them because from the CV alone it was clear they were not a fit. He then called the remaining 10% and rejected like 70% of them. A handful made it to technical interview stage, where they were rejected. And another round.
It's really hard to find a good employee, when you know who are you looking for.
Sometimes even a successful process has to go back to square one. When the finalist bails out the last minute. Or worse, they turn out to be completely incompatible with the company and let go within first 2 weeks. "Old" CVs are pulled out of the bin.
It's really easy to just click "apply" on 100 offers a day.
Some of the people that got to the phone call stage didn't even know what offer were they applying to, what is the salary and what are the remote work conditions. They've applied to, let's say a 70k job when they knew their minimum was 100k, and to a fully on-site job despite not willing to visit the office.
So probably they would take the first 10 that appear to be a perfect fit and interview them. Is this view correct?
No, usually they keep interviewing until an actual perfect fit is found. It's impossible to tell from a CV alone.
Does this mean it's better to apply to new job postings and not ones more than a couple days old?
It is. If the requirements are low and unspecific, usually the first round finds a candidate that's good enough.
Is it worth it to apply to jobs that have over 100 applicants or have been posted for few days?
Depends. If you see that the job description sounds like a perfect fit to your experience and expectations - it's always worth applying with maximum effort. "100 applicants" means only that 100 people clicked the button. Not necessarily 100 right people. Also, "few days" is usually not long enough to interview many people.
On the other hand, if your skills are generic and the job offer is generic and the money is OK - you can evaluate their chances in filling that position as high and save yourself the effort. OR - just limit your effort to clicking "apply".