Hi,
The welcome emails and the Automation trigger "When the user subscribes" only work when the user is subscribing in the frontend.
You can setup an Automation that will send an e-mail to new subscribers, but that one needs to be triggered by something else then "When the user subscribes". Instead you can setup an Automation that is time based, like once every hour to send an email to your new subscribers. To prevent existing users from receiving the email you can set a relative subscription time in the past. Dates and relative dates need to be entered in the two fields you added a question mark to.
You can, for example setup an Automation that runs once an hour to users on your list. In the left date field you specify a subscription dat one hour in the past. By doing this the email will only be send to users who subscribed in the past hour. Make sure the trigger interval and relative time are equal.
You can leave the right date field empty, because it has a different purpose. As an example you could enter January first and January 31st to fileter sibscribers that subscribed in January.
Please test your setting thoroughly, so you are sure thinks are working as you expect them to work.