As there is no exact DBMS mentioned by topic starter, I will use MSSQL as example. There is such thing as explicit and implicit data type conversion. https://learn.microsoft.com/en-us/sql/t-sql/data-types/data-type-conversion-database-engine?view=sql-server-ver16

Essentially it means that there are certain database types that database engine is capable to convert implicitly. From documentation:

Data types can be converted in the following scenarios: When data from one object is moved to, compared with, or combined with data from another object, the data might have to be converted from the data type of one object to the data type of the other.

In your case - implicit conversion takes place between integer and char.

Regards leading zeros... If you consider integer datatype, then as a numeric value - leading zeros does not make much sense.

Useful illustration regards data type conversions: https://learn.microsoft.com/en-us/sql/t-sql/data-types/media/data-type-conversion-database-engine/data-type-conversion-chart.png?view=sql-server-ver16