When you buy a house, the real estate agent or title company normally draws up a big sheet with all the costs and payments involved. There are typically two columns: one for amounts paid to or paid by the seller, and another for amounts paid to or paid by the buyer. Who is responsible for what is a legal question: this is pretty fixed.
But it's very common for the seller to agree to pay some portion of the buyer's closing costs. In any house sale I've ever been involved in, whether as buyer or seller, nobody bothers to say which costs the seller is agreeing to pay. Rather, the seller just agrees to a number. Then somewhere on the sheet of costs there will be a line that says "closing costs paid by seller" or some such wording, and then it shows a minus to the seller and a plus to the buyer. (Or something equivalent, depending on how the sheet is organized.)
The amount is negotiated. When you make an offer, you'll say whatever numbers you are prepared to offer, like "I offer to pay $100,000 for the house, seller to pay $3,000 of closing costs". And whatever other conditions, seller to repair the leak in the roof, whatever.
It makes sense for the seller to pick up some share of the closing costs, because the seller normally walks away with cash in hand while the buyer is struggling to come up with enough cash to make a down payment and pay all the closing costs, i.e. the seller probably can afford to give up some cash while the buyer may be struggling to come up with cash.
The only costs I can think of that I've had before closing day are, (a) Earnest money. (b) Inspection. (c) Credit check or application fee to bank. Earnest money is applied to the purchase price at closing, so it's pretty much a moot point. The application fee is a potential deal-breaker. I've never heard of a seller agreeing to pay this, but I guess they could. But if you can't get the loan, you probably won't buy the house, so the seller would be out money for nothing.
Everything else is normally paid on closing day. They total up all the costs and all the money floating around and at the end the seller gets one check that is the net of everything and the buyer writes one check that is the net of everything, and the realtor or title company deals with getting the money to the right people. So there's normally no issue of paying things as they come up. You do it all at once.