You have obviously not made any attempt to read the official documentation.
Searching for "concat" would have shown this as the first hit. Let me copy and paste the manual for you:
CakePHP’s ORM offers abstraction for some commonly used SQL functions. Using the abstraction allows the ORM to select the platform specific implementation of the function you want. For example, concat is implemented differently in MySQL, PostgreSQL and SQL Server. Using the abstraction allows your code to be portable:
[...]
concat() Concatenate two values together. The arguments are treated as bound parameters unless marked as literal.
There is even an example included:
$query = $articles->find()->innerJoinWith('Categories');
$concat = $query->func()->concat([
'Articles.title' => 'identifier',
' - CAT: ',
'Categories.name' => 'identifier',
' - Age: ',
'(DATEDIFF(NOW(), Articles.created))' => 'literal',
]);
$query->select(['link_title' => $concat]);