Launchbox has its own database and scraper, it will pull images and metadata when you import your roms, as long as you correctly name the platform (or set the correct "scrape as" if using a non standard system name) and your roms are not horrendously named. You can also add that metadata and artwork after the fact anytime you like by going to tools/download/update metadata/media to update every game in your library.
Or just highlight all games in a platform with CTRL-A and heading to the same tools/downloads/update metadata and media for selected games wizard to just search and grab metadata and images for that system.
As for individual games, simply right click the game and head to edit/edit metadata/media, there are various functions here including changing the emulator like you previously asked about, as well as a media download section.
The emulation tab in this menu is where you can change the emulator.
The design ethos's of Launchbox is ease of use and flexibility which is does very well, but you are coming from retrobat, which like all the systems setup based from emulationstation like batocera, recalbox etc, those are actually completely setup for you, and because of that, quite restrictive in what you can change. Launchbox is fully customizable, and to be honest is a Windows desktop program, so is going to be very different to the locked down controller driven frontends you are used too. I'm not knocking those frontends either, i personally also have a mini PC setup with batocera, and i think it's great for what it is, but Launchbox is far far more flexible.