There are companies that'll build a prototype for you. If you sell them on the idea, they might fund it to be your manufacturer. Or you use it for your purposes and license it to them to use for other purposes.
Usually you don't advertise. I hired a marketing person to help develop the story, the rollout, the collateral and messaging for the site and a crowdfunding campaign and the PR. I could have hired a PR firm, but I couldn't find one that would take responsibility for everything at anything close to a decent price. I found one overseas that would handle enough- we might still use them. They're less expensive and take 2/3 from a successful crowdfunding campaign.
I'll be doing a lot of outreach (to get users) and advertising, but I'll be doing it all through partners. They send their users/members to my site, they get certain benefits.
For instance, if you work with a major retailer, they could announce it to their customers. And creating a buzz for it on places like ProductHunt, reddit and others is something that happens through marketing. Rarely do startups do much advertising, except in small amounts to measure responses.
Another possibility is to find a specialty retailer to sell it, instead of advertising. Or find a partner to bundle it. Say you're making a special kind of wifi headphone that changes the left/right balance based on your orientation (so changes when you turn.) You might approach Bose or some other specialty manufacturer that would want to bundle your headphones with their product.