How does Java connects to sqlexpress or oracle....?

You will need a JDBC driver to connect your java app to SQL Server.

After you install the JDBC driver, you can successfully connect to SQL Server and can execute your queries.

Example:-

String sql = "INSERT INTO Users (username, password, fullname, email) VALUES (?, ?, ?, ?)";
PreparedStatement statement = conn.prepareStatement(sql);
statement.setString(1, "bill");
statement.setString(2, "secretpass");
statement.setString(3, "Bill Gates");
statement.setString(4, "[email protected]");
int rowsInserted = statement.executeUpdate();
if (rowsInserted > 0) {
System.out.println("A new user was inserted successfully!");
}