Fitness app like Fitpro

I'm looking to build an app, for iOS and Android like Fitpro (check [login to view URL]), with the same questionnaire funnel, customizable, SEO, track leads, etc. Also I need the same questionnaire for the landing page. The users will be subscribed on the website via their mobile phones through premium sms (I already have all the infrastructure and connections required for that) and on the app via in-app purchases. You can start with a prebuilt app, no problem at all, like Wicombit's Pro Fit. It could be on Flutter and the backend could be in Laravel/Node as well. The app has to support in app purchases, login via SMS/Apple/Google/Facebook and also the account created via the web funnel above mentioned. I need detailed specs based on the app mentioned.