For MPLS VPN network, usually all PEs know how to get to other PE routers via LSP tunnels. These are transport/outer tunnels/labels.
Over the transport tunnel, PE2 advertises to PE1 via BGP, to use a label L for 20.20.20.2. This L is inner label that is put on the packet leaving PE1, then PE1 also puts an outer/transport label on the packet as it exists out. All intermediate P routers just look at the transport/outer label to guide the packet all the way to PE2. PE2 knows what inner label is and guides the packet to correct interface for 20.20.20.2.
HTH.