Where to put helper classes in clean architecture

I am trying to implement clean architecture in my .net core demo app(i am creating rest api) .
I have created some helper classes for mail sending,file processing and some dapper helper classes.
I am not using entity framework. I am wondering where should i put my dapper helper classes ? should i put in persistance layer ? Wondering if my project will grow there will be many supporting classes, in which layer should i put my helpers ? Also if somebody explain me what exactly infrastructure layer will contain when creating rest api.
I found one great explanation here http://www.atrenko.com/blog/2018/05/03/clean-architecture/
but still confused about my dapper and mail helpers and others.
Can anybody explain me which layer is the best place for my common helpers ?
Thanks