When a source node sends a packet to a unicast destination, what will be the entry in the frame destination address(layer2 mac address)? will it be default router's mac address?
It's depends whether the destination is within the subnet or not. If it's within the subnet, it'll put the destination device's MAC itself (assuming ARP has already done it's job). If the device is outside the network, then packet will be forwarded to the default gateway, hence, the MAC address of the default gateway.
When a node wants to send a packet it will look up the destination address in it's routing table. Based on that table it will determine the IP address and interface for the next hop. If the destination is on one of the local subnets then the next hop IP address will simply be the destination. Otherwise it will be the IP address of a router (most likely the default gateway).
The MAC address (if applicable) for the next hop IP address will be determined by arp (for IPv4) or neighbour discovery (for IPv6)