how to keep an app open while working

Marty410 wrote:

working on an issue: using a calculator app but sometimes I'm doing something else (making notes etc. I'd like to keep the app open rather having it close.

When you switch from one app to another, you're not actually closing it.

Switch apps on your iPhone, iPad, or iPod touch - Apple Support

But there is no way to keep both apps visible on an iPhone screen.

To keep an app open while working on other tasks, you can use Split View or Slide Over on your iPhone. Swipe up from the bottom of the screen to enter the App Switcher, then tap and hold the app you want to keep open. Choose "Split View" to use two apps side by side or "Slide Over" to have one app floating on top of another. This way, you can continue using your calculator app while making notes or performing other tasks.

MacGuru180 wrote:

To keep an app open while working on other tasks, you can use Split View or Slide Over on your iPhone. Swipe up from the bottom of the screen to enter the App Switcher, then tap and hold the app you want to keep open. Choose "Split View" to use two apps side by side or "Slide Over" to have one app floating on top of another. This way, you can continue using your calculator app while making notes or performing other tasks.

You seem to have mistaken directions for using an iPad for something that will work on an iPhone. Neither Split View nor Slide Over are available on an iPhone.