how to scaffold the database triggers
It's not possible to scaffold Triggers, and it doesn't make much sense since they are a database specific type of object (not sure what they could map to in C#?). They don't directly describe the database model, as opposed to Tables, Columns, and Keys. What is your end goal that you wanted to scaffold the Triggers?
Please see Entity Framework Core - Reverse Engineering documentation for more information.
Particularly the How it works section details what is scaffolded from the database:

Reverse engineering starts by reading the database
schema. It reads information about tables, columns, constraints, and
indexes.
Next, it uses the schema information to create an EF Core model.
Tables are used to create entity types; columns are used to create
properties; and foreign keys are used to create relationships.
Finally, the model is used to generate code. The corresponding entity
type classes, Fluent API, and data annotations are scaffolded in order
to re-create the same model from your app.