Not a published author but have worked work for a large publisher. The process is that you get a literary agent who essentially acts as a broker between you and publishers and tries to sell your book.
And the publisher acquiring the book depends on a number of factors - their strategy for that quarter/year, what sort of books they are interested in, whether or not it’s like other books they have acquired recently (e.g. they might want to capitalise on a trend if your book is like others, but won’t want to publish too much of the same thing), if it’s something new and exciting, if it’s likely to win awards etc. It’s a real talent for commissioning editors to spot something special and try and predict if it’ll do well or to take a risk if it’s something a bit out there. Obviously they don’t want dull, generic books - but then some of those are very popular and exactly what people want. And then maybe it’s something very different which could create a lot buzz - but if it’s too out there it might not be worth the risk.
It really depends on the publisher and what they’re looking for, normally they’ll have their own brand and want books as an extension of that - so that’s where the agent’s knowledge of each publisher and what they like is very valuable.
If the manuscript is really exciting you might get a bidding war between publishing houses which obviously helps negotiate your contract.
The agent will broker your deal and you’ll often get an advance and a percentage of sales depending on loads of factors. But normally it’s the big names or those with a lot of hype who get the big advances. In a way you don’t want too big an an advance as there’s a lot of pressure to make up the cost in your sales, and if you’re sales don’t reflect your advance it might mean you’re in a harder position to negotiate your next book if you get another published.
That’s the gist of the acquisition but obv overall it’s worth quite long winded with lots of steps for marketing, editing, sales, royalties etc!