Storing data for cross platform use?

Hi there!

*Very newbish developer question preface

I'm currently starting my first real coding project by creating a basic crm platform for my company. Thinking down the line, the crm to first be accessible via web/browser. Later on, I'd like to develop an IOS/android app for our employees and clients to use later on. The app will receive the same data (such as user posts and images from the client or client contact information from the employee) as the website version.

I plan on using mysql and nodejs for the database- however,

• What is the best way to store the data entered by users for cross-platform use?
• Do I have a misunderstanding on how databases work to begin with?
• Will the posts and images even be stored in the mysql database? If not, how will they be stored?
• Can anyone recommend a good article or tutorial on setting up databases to receive user entered data and associate that data with the user?

I greatly appreciate your help on this!