What is the best way to develop an Android / iOS app?

First and foremost, I would challenge you to test the user experience and prove the concept of your product before building a mobile app. Mobile app development is not cheap and if you get the feature set wrong from a user standpoint, you will have spent money to develop blindly. Can you build a similar experience that provides value in a web application instead? You probably can. Even a few hundred users can get you the information you need to know almost exactly what the mobile app needs. There are mobile first websites that were around for years before they built the app just to make sure the experience was on par with what they expected. Test, test, test, and test again before jumping out there.

I run a mobile development agency for the last 15 years. So I am a little skewed.

Before you start looking for somebody to get the app developed, start with very detailed specs. The more detailed it is the better. After that start creating the user flow. There are tools like invisionapp.com to create good app screen mockups. another decision that would impact is going going to be which platforms you want to create the app, native is, native Android or both or or a hybrid app like react native. In your case, a hybrid solution might be better if you want the app to support iPhones and Android users.

You get some high level estimate using some tools like https://www.app-estimator.armia.com/ . this would help to get an idea what to expect when you talk to developers. Get few proposals from freelancers and agencies.

Good luck with your app

Hello! I will start with how much budget you got. How much time and resources are you willing to spend on the app.

Then decide on the technology/platform to build the app for. If you are not technical person then Clarity is best place to ask for such technical questions or find some technical person or ask online.

Plan your minimal viable product (MVP). It should have the most basic required features for the app to be functional.

Going with freelancer will give you are jump start to the app development with MVP at lower cost. Quality can be questionable as freelancers usually work on multiple projects. But this can be overcome by filtering right candidates with right qualifications.

Once MVP is ready and gaining traction then you can decide on further investment.
If it is looking really good opportunity you can contact an agency and they can take you app from the MVP to the next level by designing, structuring and polishing the app.
Benefits of agency is they assign dedicated developers to the development of your app and have lot of experience in designing, developing and maintaining the apps. They also have dedicated UI/UX person. They know about best practices and help in getting valuable metrics. The only down side is cost. It can get expensive.

Once the app is profitable you can also consider moving development in-house or hire own developers who will work on the app close to you. You will have all the control, guarantee quality of the app and keep working on it continuously.

So freelance for jump start, agencies for laying out foundation and polishing, and then hire developers for continuous development and maintenance.

I hope this helps.

Plan in detail for MVP (Minimal Viable Product). When you are bootstrapping and you will be outsourcing the development, it is best to do it in iterations.
When you look for an agency/developer/s see their portfolio, identify previous customers and contact them for feedback.

Best to have it through referrals. I am 15y+ in the industry, and when someone asks me to refer a developer or an agency, I always tell risks, as all people have strong and week sides. I usually prefer not to refer people, unless I am very positive about the outcome, as I will have to put my name to the connection, and follow up the execution.

When you have a contract for the development, make sure you have a long term support clause. Normally it is an % of the base software value paid annually. You should have this at least for 1-2 years for each feature you produce to keep the product stable. So make sure you have this in your budget!

I suggest you talk to a professional, before stepping into long-term relationships. You need to know what you will be producing, how are you going to distribute it. Do you really need an Android/iOS app. E.g. we have a mobile app SaaS business, and after having apps for Android/iOS we decided to go with a PWA app technology, and just dropped the native app support, as it was much faster to install, as our users were coming from our website. But if you rely on the App-Store listings, that's another story, bust still you can do a hybrid app. It is a decision that might increase your costs drastically if you want to pivot once the product is completed.

I advise you to do all technical specifications in detail, including the mock-ups first. Then do the design, then proceed with development.
This can be done either by the agency you hire, or in advance by an advisor/professional in the field, and based on that find the best agency.

Never start development, before it's all figured out in advance!

Let's be honest. Outsourcing is the most smart decision to address this problem and our company already want to cooperate with you! 10+ years in Hi-tech business have resulted for R-Style Lab in multiple knowledge-intensive projects for customers. So just contact us to jump-start your own business idea :)
Here is our website: https://r-stylelab.com/services/internet-of-things-app-development