Some tops have hidden support built into them. The most common type is called a shelf bra (not to be confused with a shelf bra that is a cup-less bra). A second layer of fabric is sewn underneath the singlet that stops just under the bust and is pulled in with a line of elastic (see illustration below). There are no separate cups, the support comes from the stretchy fabric and the elastic that goes under the bust.
This kind of top can be worn without a bra and is great for lounging or sleeping in. You can also wear them out but be careful as they do nothing to hide they shape of nipples (nipple covers can help with this). You can also wear them with a bra to give extra support, it’s common to combine it with a sports bra to minimise movement while exercising.
Some clothing will have built-in support that is different to a shelf bra. Contoured foam cups may be sewn into tops and dresses so they can be worn without a bra. Some will even use underwires so it’s essentially like having a bra sewn in. This is quite common in swimwear. It is also a solution for special outfits or in costumes where you wouldn’t be able to wear a bra without it showing.