Azure DevOps wants you to provide secrets to pipelines using a so called pipeline library. You can store single line strings as secrets in the pipeline library. You cannot though store multi-line strings as secrets without messing up the line-breaks.
Storing multi-line secrets as “Secret Files”
If you need a multi-line secret (e.g. an SSH private key or a kubectl context) you need to provide this secret as a secret file. When you open the library click the 2nd tab ‘Secret Files’ and upload your secret.
When accessing secrets and secret files via variable in the pipeline there is no difference in using the secret variable!