9-to-5 here is figurative. No one is tracking hours for software engineers at most companies. It just means: you start working in the morning, and you stop working in the evening. As opposed to working until late at night and/or weekends.
Lots of people arrive before 8 but it's not the norm. 8am meetings --before COVID-- were always generally frowned upon.
In my experience, lots of engineers will show up far after 9 AM and leave well before they have reached a full day of work. Its a very privileged system that exists because it is so hard to hire engineers. At least for now.
If you come in at 9am, do work, have lunch, make coffee, work more, suffer meetings, work, chat at the water cooler, work again, and leave at 5pm, you're working 9-5.
Most people are in the office (when they are actually in the office, which isn't often these days), from 9ish to 5ish. I've worked at many companies that offered dinner at 7 to encourage people to stay late, but that only seems to have the effect of having people start later. No one really tracks hours as long as the work is getting done. Plenty of time I'll cut out of work around 2pm for something and then check in later in the evening for a couple hours.
General trend is people in their 20s work longer and later. Most people over 30 at any tech company I've worked at are out the door before 5. But also people over 30 tend to more reliably be in the office at or before 9.