For this specific example? Just like any company's 95k. They got the initial capital from investors, they sold things to customers, they got loans from a bank, etc.

Or you're asking generally where money comes from? It's a good (and complicated) question. Google monetary and banking. In multiple senses, money DOES come out of thin air.

City budget or loans. Easy enough if your mayor have a friend who want to start a gardening company !

I'm joking: even without corruption, you can find an ideological reason, as a company will surely be more efficient than public servants!

But more likely because of incertitude: what do we do if a gardener resign? Paying an existing company 95k/year instead of spending 75k/year directly isn't a huge expense increase, and if 20k/year is the price for peace of mind (no new equipment to buy, no HR issues...) it can very well be worth it.

I'm not saying this is good or bad by the way, i'm saying this is how GDP work. It's factual. Yes, there is a left-wing slant about how i presented it, but it wasn't heavy, and a liberal could use the same example in the same way and justifying a better distribution of work and concerns (while still finding that GDP is worthless in this case)

This is interesting as an indicator, it's meaningless as a target.

But borrowing that money has implications for the economy and other businesses.

This is the mistake the OP made. You can't "goose" GDP by borrowing because that money has to come from somewhere. And the money used to "goose" GDP is money that doesn't go to other productive uses.