declare Usage

IF 1=1
BEGIN
   DECLARE @test VARCHAR
   SET @test='1'   
   PRINT 'in if:'+@test
END
GO
PRINT 'out if:'+@test
 

IF 1=1
BEGIN
   DECLARE @test VARCHAR
   SET @test='1'   
   PRINT 'in if:'+@test
END
PRINT 'out if:'+@test

After comparison, we can see

Scope declare a variable is a batch where, if not blocking its scope, that the above code if we value both inside and outside of the code in a batch, so @test are available and if they contain also in.

GO is the end for a sql statement