You can create a new document based on the current contents on the clipboard. A 'paste as new document' command allows you to start a new project from a range of sources, e.g. from another project in Affinity products or from copied or captured imagery.
If you've copied a vector object to the clipboard, then the command will create a new document that is sized to the vector object. The copied object will remain as a vector object in the new document.
If you've copied a drawn pixel selection, the command will paste those pixels into a new 96dpi document in a pixel layer.
If you copy images from Internet web pages or capture a screenshot to clipboard (e.g., using Apple's Screenshot app), the new document creates an image layer, not a pixel layer, from the copied data. The advantage being that the layer is non-destructive, i.e. it avoids rasterization and loss of the image's original resolution and color space. You can still manually rasterize by right-clicking the (Image) layer and choosing Rasterize.