Which permission should I use for a role to see a shortcut to Custom Block Library?
The permissions defined for the module are:

administer shortcuts:
  title: 'Administer shortcuts'
customize shortcut links:
  title: 'Edit current shortcut set'
  description: 'Editing the current shortcut set will affect other users if that set has been assigned to or selected by other users. Granting "Select any shortcut set" permission along with this permission will grant permission to edit any shortcut set.'
switch shortcut sets:
  title: 'Select any shortcut set'
  description: 'From all shortcut sets, select one to be own active set. Without this permission, an administrator selects shortcut sets for users.'
access shortcuts:
  title: 'Use shortcuts'



To see the shortcuts you need the permission Use shortcuts.
To see the shortcuts in the toolbar you need the permission Use the administration toolbar
To see the shortcut for the page /admin/structure/block/block-content you need the permission Administer blocks