how to create a link to open app? [closed]

you can have different cases like whatsapp provide a custom so you need only link to this
https://faq.whatsapp.com/425247423114725/?cms_platform=iphone&helpref=platform_switcher

or calling a app direct instagram://user?username={USERNAME} as link

Your question has been answered here already https://stackoverflow.com/questions/48787496/opening-instagram-app-on-mobile-and-url-on-desktop