There is a vast amount of confusion in the question
Suppose A is trying to start a startup. A has not registered a company yet. A wants to hire B. Will B agree to work for an unregistered company? If no how could A hire B during the initial phase?
Again, there is vast confusion here,
It takes like "an hour" and a few pennies to register a company.† So the whole question is a bit pointless and bizarre.
Sure, it's totally commonplace that on Monday, A will say to B "I want to hire you for MegaApp! The company will be registered Thursday! Are you in?!" And A will say "Sure! sounds great!"
Let's say B goes to do a contract or salary job for Microsoft or Apple. Say B starts on the 1st. The pay dates may be the 15th or whatever. Of course, obviously, B "trusts" the established company until the 15th. In contrast...
Let's roll back to point 2. A is just a "normal person" doing a startup, not an established company. So A wants B and B says "Yes!" Hooray! What B will literally say is "Yes! Here's my wire transfer details." (Or ACH, or whatever.) Only a fool would start work at / trust some person with a startup idea, and begin working without money. (If you do hire such a fool, they will be a useless programmer.)
Sure. B may start work for like "a day" until the advance payment arrives. I've often worked for well-known entrepreneurs or the like who I have a long relationship with, and I'll give 'em like "a week" to wire money for some project; set aside such exceptions. In startups you have to pay in advance, so, again, the whole premise of the question is sort of pointless.
Again, harkening back to point 2, sure, you could say to someone, on Monday, "Great! Let's begin! It's after lunch so I'll wire you $5,000 in the morning, and, the company will be registered by Thursday! How's that sound!"
It's completely commonplace that someone, A, doing a startup, at first (perhaps for a few weeks - whatever) just personally makes payments, for example to programmers. Once everything gets rolling and the paperwork is done, etc, someone just sorts out the accounting. For that matter it's perfectly legal and OK to pay to have some MegaApp idea developed, just "as a person" (naturally, this would be a business expense like any other self-employed expense).
Overwhelmingly, in the sort of situation you describe, programmers and designers just work for you as freelancers, ie you just send them money to do things. You're not going to put them on payroll. So it's not complicated.
Unrelated tip -
You mention "technical" software. "Technical" software is Just Like Normal Software but, say, 5 times more expensive.
† Note that anything, whatsoever, you do with software, will cost say $150,000 absolute minimum. So even if it's going to cost "a whole" 5k for legal (for some reason), that is so little money in the world of startups, it means nothing.