You got it right for the 1st part, but all Derrick booms on tall buildings were bought down with smaller Derrick booms, unless for short buildongs. This is a common practice in Hong Kong.
i think they assembly the yongmao stl230 for dismant scm d228b and then derrick boom were set up on top to dismant yongmao stl230 and mobile crane was set up to dismant the derrick boom from that building