Action<OptionsBuilder> vs Action<Options> pattern?

What are the differences between these?
Actually, Action<Options> can be treated as a builder to the Options. And providers can provide extension methods for Options type to hide the complexity.
Why DbContext configuration uses Action<DbContextOptionsBuilder>?

You're right in that the optionsAction in

IServiceCollection.AddDbContext<TContext>(Action<DbContextOptionsBuilder>? optionsAction)

could be an Action<DbContextOptions> instead.

However, the "configuration action" (by lack of a better term, this is not the "options pattern") of Entity Framework Core is vastly more complex than your average Dependency Injection configuration method.

You're usually not done with setting a string or two (as with authentication cookies) and perhaps an action or a converter (JSON comes to mind), but there are thoroughly complex configuration scenarios for Entity Framework Core.

Therefore, as is documented in the DbContextOptionsBuilder class, it:

> Provides a simple API surface for configuring DbContextOptions. Databases (and other extensions) typically define extension methods on this object that allow you to configure the database connection (and other options) to be used for a context.

It hides all of the plumbing required for that extensibility inside the builder, as opposed to on the DbContextOptions itself. The builder plumbing is only required at construction time, while the options themself, once built, are used at runtime.

See also MS Learn: DbContext configuration and initialization: DbContextOptions:

> These Use* methods are extension methods implemented by the database provider. This means that the database provider NuGet package must be installed before the extension method can be used.

And:

> Optional configuration specific to the database provider is performed in an additional provider-specific builder. For example, using EnableRetryOnFailure to configure retries for connection resiliency when connecting to Azure SQL

Everything that happens in the Use* methods is used to construct a DbContextOptions. You wouldn't want to do that yourself, and you wouldn't want to have an Options class that contains too much plumbing only related to its setup, not to be used after construction.