I think you're conflating a couple things. First, Filestream allows access to the filesystem for work with C# (or other .NET apps) and will work more efficiently. You can just use T-SQL, but you lose some efficiency from the file level operations. To use T-SQL, you just run an insert.
create procedure insertintofs
@pk int,
@fs varbinary(max)
as
insert into fstable (pk, data) values (@pk, @fs)
?
https://www.red-gate.com/simple-talk/sql/learn-sql-server/an-introduction-to-sql-server-filestream/