Custom content for Sims 4 will be in package files, with the exception of Sims and Lots. So how to install Sims 4 creations depends on the type of creation.
All custom content in Package files need to be placed in the Sims 4 Mods folder. The location of this folder is usually:
Documents/Electronic Arts/The Sims 4/Mods .
Recolors show up as a variation of the original item with no specific indiciation of being Custom Content. This is for recolors of original objects. You must search for the original item and then the recolor will show as a swatch of color.
Downloaded Sims and Lots
Downloaded Sims and Lots contain each several files. All the downloaded files for that creation need to be placed in the Sims 4 Tray folder. The location of that folder is:
Documents/Electronic Arts/The Sims 4/Tray
You must unzip the file and place only the tray items in your tray, not the zip file.
Once you have placed them there, you should be able to find the Sims or Lots in your personal Gallery in the game.
Mods allow players to add nearly anything they want to The Sims 4—and this includes stuff that could be considered to be on the more on the taboo side of things.
Under normal circumstances, you can only pursue certain types of relationships with certain people. If a sim is related to another sim, they can't romance each other—and they definitely can't marry each other, for example. Another rule: teens can't become pregnant. But just because the game is designed a certain way doesn't mean players don't want to toy with certain simulations anyway, regardless of how socially acceptable they'd be in real life.
That's where a number of curious mods by BIlly Rand come in. Rand is a modder who specializes in 'forbidden' types of mods—during the last couple of days, he's uploaded videos which showcase teen pregnancy, incest and even polygamy in The Sims 4: