Correct, if the length is not explicitly declared, a VARCHAR type variable is defined as being one byte long. 🙂
https://docs.microsoft.com/en-us/sql/t-sql/data-types/char-and-varchar-transact-sql?view=sql-server-ver15
varchar [ ( n | max ) ] Variable-size string data.
When n isn't specified in a data definition or variable declaration statement, the default length is 1. If n isn't specified when using the CAST and CONVERT functions, the default length is 30.