  We identify hierarchical structures in the Vela OB2 complex and the cluster
pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine
learning algorithm StarGO. Five second-level substructures are disentangled in
Vela OB2, which are referred to as Huluwa 1 (Gamma Velorum), Huluwa 2, Huluwa
3, Huluwa 4 and Huluwa 5. For the first time, Collinder 135 and UBC 7 are
simultaneously identified as constituent clusters of the pair with minimal
manual intervention. We propose an alternative scenario in which Huluwa 1-5
have originated from sequential star formation. The older clusters Huluwa 1-3
with an age of 10-22 Myr, generated stellar feedback to cause turbulence that
fostered the formation of the younger-generation Huluwa 4-5 (7-20 Myr). A
supernova explosion located inside the Vela IRAS shell quenched star formation
in Huluwa 4-5 and rapidly expelled the remaining gas from the clusters. This
resulted in global mass stratification across the shell, which is confirmed by
the regression discontinuity method. The stellar mass in the lower rim of the
shell is $0.32\pm0.14$ $\rm M_\odot$ higher than in the upper rim. Local,
cluster-scale mass segregation is observed in the lowest-mass cluster Huluwa 5.
Huluwa 1-5 (in Vela OB2) are experiencing significant expansion, while the
cluster pair suffers from moderate expansion. The velocity dispersions suggest
that all five groups (including Huluwa 1A and Huluwa 1B) in Vela OB2 and the
cluster pair are supervirial and are undergoing disruption, and also that
Huluwa 1A and Huluwa 1B may be a coeval young cluster pair. N-body simulations
predict that Huluwa 1-5 in Vela OB2 and the cluster pair will continue to
expand in the future 100 Myr and eventually dissolve.
