OOP in a nutshell.
You have a class to model a clock. Everything you need to know about a clock is in this class, it has a few routines that allow you to set the time, and display the time. This is Encapsulation.
But now you want a digital clock. This is a clock like you had before, but it wants to display the time in a digital rather than analog display, so you derive a new class off the original and replace the display routines to show stuff in digital format. This is Polymorphism.
And now you want an alarm clock. This does everything clock does, but it also makes a noise at set times, so you derive from the original clock class and add new routines to set and ring the alarm. This is Inheritance.
And there you have the 3 fundamentals of OOP.
Note that in all cases, you still have a clock. This is how OOP works, that you can sell someone a clock, then hand them an alarm clock and they will still be able to do 'clock' things to it like setting and telling the time.
This is the lifecycle of making software and delivering it to the customer. You start by figuring out what it is they want, figuring out how you're going to go about it, figuring out how to code it, figuring out why it doesn't work quite right, and then figuring out how to install it.
There's no "right" answer to "what is SDLC", but to show that you understand all aspects of creating software that the customer uses. Its to show that you understand there's more to software development than simply coding.