They would quit working at the old company before they start work at the new one. usually there wouldn't be overlap.

You are right. You cannot onboard a new job before you leave your old one. Accepting an offer is not part of the onboarding process though. It happens before.

After an interview process the company makes an offer. The candidate can then accept or reject it. But that is really all informal. You can then negotiate with them for an official start date and contract. You just need to ensure you can hand in your notice and work the rest of your notice period before the start date of your new contract.

I don't know anyone that would hand in their notice before accepting the initial offer of a company. At least here in the UK.

If you begin the onboarding process as a senior backend engineer without inquiring as to what your working environment will be, then you're just incompetent.

Is this some usa user who haven't heard about other countries? (and I doubt it's even true in the usa).

How are they going to use a personal device when corporate policy locks that down?

They don't use a personal laptop, and I've never heard of such thing for any company that has more than 10 employees. The security risk is huge

Clients will have intellectual rights on anything produced for them. Removal of that data from their systems and storing it elsewhere will be a violation.

Using your own equipment other than maybe your monitor, mouse, or keyboard will be a no go. I don't know of any serious workplace that would let you do otherwise.

Even if you are a self employed contractor you will need to remote in to their virtual environment and work in that.

Never, ever, EVER use your personal equipment for work.

There are a ton of legal reasons for that, not just around who owns the Copyright of work done on that machine as well as licensing of the software running in it (most commercial software has different licensing conditions for personal and commercial use) but also because if there's some kind of legal case against that company your equipment might very well be confiscated as part of an investigation.

Also, more in general, if you have personal practices which are legally dubious or often frowned upon (piracy, porn) you don't do it in the same machine where you're doing your professional work, definitelly not on a work machine but even in your own machine it's risky (see the point above about how your machine might end up confiscated and examined by the authorities if the company is investigated). The principle of "you don't shit were you eat" applies here.

Even for your own company, it's best to have the company stuff separate from personal stuff.

Beyond that, it's also a very good idea in terms of having a good work-life balance to separate the personal from the professional: ideally you keep a very strong separation between work and not-work, at all levels, from work time and outside-work time to work/personal machine and work/personal phone - it helps make clear both for yourself and, even more importantly, others, that there is no work outside work, which reduces the chances of management doing things like call you on weekends or evenings with questions and makes it easier for them to accept when they try it and you say "I'm not at work now, so I'll pick this up first thing when I'm back at work" - the cleaner and harder the split the less room there is for the "barely in control, almost 100% reactive" kind of manager to sneak work stuff into your personal-time.