If you seek some kind of Certification derived from professional Training, you could check
into the Apple Training, and Certification; as available in certain locations...
• Apple Training and Certification:
https://aatcportal.apple.com/AATCWeb/locations
• Apple Legal - Training and Certification - Agreements and Policies:
https://www.apple.com/legal/sales-support/certification/
Not sure where the over-lap where training & certification may be helpful in another brand
environment; unless you were working in a business where several brands of products were
in use, or being sold, where customer service may be enhanced given additional experience.
While only glancing through the above portal into several of links inside, I did not see an iOS
or other section; those topics could be asked a training center or official Apple Store. In some
regions, training, certification, consulting products may be available by independent companies.
To see what may be available in a region or country, some of these appear in the Locator page
for support, sales, repair, training, consulting: Apple Find Locations: https://locate.apple.com/
or, Apple - Find Locations - Country Selector: https://locate.apple.com/country
You may be able to visit an Apple User group in your area to ask what is available if you join;
also you could ask an authorized reseller/service provider that may offer free or low-cost
product introductory training, as the official retail Apple Stores have done every so often.
Contact an Apple Store, & ask about free training they offer customers. Most have an online
schedule of topics they have mini-courses on. Even the independent stores do sometimes.
In any event...
Good luck & happy computing! 🙂