In QGIS 2.18 there was plugin "Copy layers and groups to clipboard". It was allowing just in two clicks to copy/paste layer(s) with style and etc. from one project to another.
Is there something similar in QGIS 3.0?
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You can drag and drop layers or groups between QGIS windows.
Note, this may not work reliably. As commented by QGIS developer ndawson - "it often results in a kind of "corrupted" layer, which has no features and can't be styled". Though I personally have not come across this issue.