How to select text with touchpad without pressing the "hard" key?

The nearest thing available is the "Three finger drag" option in System prefs -- under trackpad you'll find the option to use it:

This lets you drag by hold three fingers on the trackpad and dragging..

The settings location where the three finger "drag" is enabled has changed over time.

From OS X 10.11 "El Capitan" to macOS 10.14 "Mojave":

1. Go to System Preferences > Accessibility
2. Choose Mouse & Trackpad from the left options list
3. Click Trackpad Options
4. Tick "Enable dragging" and select “three finger drag” from the drop-down menu next to it

For macOS 10.15 "Catalina" and later:

1. Go to System Preferences > Accessibility
2. Choose Pointer Control
3. On the Mouse & Trackpad tab, click Trackpad Options
4. Tick "Enable dragging" and select “three finger drag” from the drop-down menu next to it