LED strings are almost always in series to get the total voltage of the LEDs near the supply voltage.
For instance, 12 V strings will have three LEDs in series, and a resistor to control the current.
LED lamps that run at 220 Vac contain rectifiers and many, many LEDs in series to get a large voltage. There are various different methods used to control the current. It is rare to have 220 Vac directly connected to LED strings. There is often a control box somewhere.