  In this paper we introduce the following new ingredients: (1) rework on part
of the Lagrangian surgery theory; (2) constructions of Lagrangian cobordisms on
product symplectic manifolds; (3) extending Biran-Cornea Lagrangian cobordism
theory to the immersed category.
  As a result, we manifest Seidel's exact sequences (both the Lagrangian
version and the symplectomorphism version), as well as Wehrheim-Woodward's
family Dehn twist sequence (including the codimension-1 case missing in the
literature) as consequences of our surgery/cobordism constructions.
  Moreover, we obtain an expression of the autoequivalence of Fukaya category
induced by Dehn twists along Lagrangian $\mathbb{RP}^n$, $\mathbb{CP}^n$ and
$\mathbb{HP}^n$, which matches Huybrechts-Thomas's mirror prediction of the
$\mathbb{CP}^n$ case modulo connecting maps. We also prove the split generation
of any symplectomorphism by Dehn twists in $ADE$-type Milnor fibers.
