Help me understand this statement PLEASE

Hi there!
I am a newbie. I want to understand what this statement means in step by step

PCMSK0 |= (1 << PCINT0);

I know it enable the PCINT0 bit in PCMSK0 register. but I don’t know how it works?
I know how the bitshift operator works but can’t understand particular statement?

Set the bit that corresponds to INT0 to 1, keeping all other bits.

PCMSK0 = PCMSK0 | (1 << PCINT0);

(1 << BITNO) converts a “bit number” into a “bit mask”, ie “3” becomes 0b1000 (a 1, shifted left three bits.)
“|=” is like “+=”; “a |=b” is the same as “a = a | b”
Oring a bitmask with a variable sets all the bits that are one in the mask, without changing any of the other bits. ( 0b10100101 | 0x1000 yields 0b10101101.)