I did reverse engineering as a hobby in my teens. The method for 90% of cracks back in the day boiled down to:

1. Launch the app.

2. Trigger the "not registered" screen.

3. Search for the string in the binary.

4. Search for code that references the location of the string and squint hard until you spot the "check if CD-key is valid" section.

5. Patch the final opcode from "jnz" to "jz". If you felt fancy, reverse engineer the checking algorithm to create a keygen instead of a patch.

Anticracking methods became mainstream in the late 90s, but they weren't very sophisticated and most amounted to obfuscation.

Long live +ORC, +Fravia and CracksLatinos.