You cannot "Embed" SQL Server database in your DLL. MS SQL Server itself is a quite big service application itself, which can host/serve/manage many databases, while you could install it (sat the Express version) on each computer where your Acad app runs, it usually runs on a server computer of your office's intranet, or even from a remote host in the cloud. Your app can only access the database via the SQL Server. Further more, with current technology trend, letting client side app (your CAD app) to have direct access might not be a good practice, and your IT department may even not allow such direct access. Instead, the data served from SQL Server (or any database server) would be wrapped with service and exposed to end apps, most likely via HTTP(s) services.
Anyway, if your app need to access the data hosted by SQL Server, you DO NOT embed SQL Server with your CAD app DLL (not possible!). You use database access API, ADO.NET, basically, such as System.Data.SqlClient (Microsoft.Data.SqlClient, if you do Acad2025) in conjunction of other classes in System.Data.Xxxxx namespaces, if you do have access to a Sql Server running somewhere in your network, or even in your computer (then all your users have to go to your computer to get data!). You may need to learn some basic about how to set up database, assign permission to access... which are the knowledges/skills of another profession of database server administrator.
You can search the net/youtube for ADO.NET tutorial - there are tons of it.