How a circuit is wired will determine the voltage and current needed to properly operate the circuit. Below as an example you will see two circuits using five 160-1445-1-ND LEDs, which are a 2V 20mA rated LED. As you can see there is a drastic difference in the voltage and current needed to properly operate each of these circuit types.
Parallel components have the same singular rated LED voltage across the entire load, but the current requirement will increase or decrease based on the number of LEDs.
Series components have the same singular rated LED current through the entire load, but the voltage requirement will increase or decrease based on the number of LEDs.
However, generally speaking, most LED lighting use a series-parallel combination.
Series-parallel combinations are the best of both worlds, where they get to keep some of the current consistency benefit of series wired LEDs but are also able to keep the lower voltages of parallel wired LEDs. The series branches help produce a more consistent current, which helps reduce thermal runaway, and increase LED longevity, and lighting consistency, while the parallel branches keep the voltages lower, which is easier to regulate along with added safety benefits.
Ideally, for reliability and lighting consistency, it would be best to have LEDs each wired in series to a constant current driver. This is usually impossible for long strips due to the very high voltage requirement of the LEDs, but also if one of the LEDs were to burn out then the entire strip would not light, where with combinational series-parallel wiring, only one branch of the strip would not light which will still leave some lighting.
Keep in mind that the remaining LEDs may be damaged when an LED(s) burns out, gets damaged, or is cut from a strip in a parallel or combinational series-parallel constant current LED driver circuit, as the fixed current may be too high for the remaining LEDs. This is yet another reason constant voltage drivers are easier to work with and sometimes preferred over constant current drivers for parallel or series-parallel combinational circuits.
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It's not a good idea. Look how a (generic red) LED conducts current when you apply a voltage to it: -
At 2 volts, the LED is taking 20 mA. If the LED was manufactured slightly differently it might require 2.1 volts or maybe 1.9 volts to push 20 mA thru it. Imagine what happens when two LEDs are in parallel - if they "suffer" from normal manufacturing variations, an LED that only needs 1.9 volts across it would hog all the current.
The device that needs 2.1 volts might only receive 5 mA whilst the 1.9 volt device would take maybe 35mA. This assumes a "common" current limiting resistor is used to provide about 2 x 20 mA to the pair.
Now multiply this problem out to 8 LEDs and the one that naturally has the lowest terminal voltage will turn into smoke taking the best part of over 150mA. Then the next one dies then the next etc...