The Intrusive Import
macOS is heavily integrated with the Apple ecosystem, designed to make media management as seamless as possible. Because of this, it exhibits a highly aggressive default behavior regarding external storage. Whenever you plug your iPhone into your Mac to charge it, or insert an SD card from a digital camera, macOS instantly assumes you want to import your pictures. It will automatically launch the heavy, full-screen Apple Photos application, interrupting whatever you were currently working on and demanding your attention to manage media you likely had no intention of importing at that exact moment.
Blocking the Auto-Launch
To maintain your focus and stop macOS from automatically launching the Photos app every time you connect a device, you must revoke this specific permission using a hidden terminal command, or configure it per-device.
To fix this globally so it never happens for any device, the fastest method is via the terminal. Open the Terminal application (found in Applications > Utilities). Type exactly: defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES and press Enter. The change is immediate and silent. If you prefer a graphical method, you have to do it per device. Plug your iPhone in and let Photos open. In the Photos app, look at the top-left corner just below the toolbar. You will see a small checkbox labelled Open Photos for this device. Uncheck that box. You will have to repeat this manual unchecking process for every unique SD card or phone you ever connect.