This feature is only available in the desktop editor.
With both articles open you can copy select the content blocks you want to copy and use Edit > Copy to copy the content blocks to the clipboard. Now switch to the new article and use Edit > Paste to paste the content blocks.
If you want to copy the content blocks, hold down the OPTION (Mac) or SHIFT (Windows) key while you drag the steps.
The content blocks are moved to the new article. In the screenshot below the content was copied from one document to the other.