You can’t have in-app scheduling in Adalo (unfortunately); there is no server-side scheduler, and if you set up a timer in the app it will stop working when app is closed.
With the Push Notifications API you could send Pushes from outside, but according to official documentation you can’t send it “in bulk”, only one-by-one (to a single address).
So in your case I’d suggest using Integromat, with the following logic:
list all records from Adalo, from Users database
add Iterator to proceed this records
add HTTP request to send Push notification to the user email
schedule this action to run once a day.
With a large userbase this could be costly from credits perspective.
Please keep in mind: each day you’ll use 1+2N credits, where N=number of users. Which will convert to 30+60N credits per month.
For 100 users, it will be 6K+ credits. This is beyond free plan.
So, I’d advice to have a look at the Integromat plans to avoid unpleasant surprises