What kind of memory it is? I was reading about various memory techniques : mnemonics, memory space, peg system, space repetition etc . They are used when I have to memorize something by myself for longer duration.

There are many other memories which are in my mind but I dint memorize them by myself. They are automatically stored. Like : my travel experience, some childhood activity etc . Some of them are faded some are still very clear. I can visualize them clearly.

But there are some memories, I cant visualize them. But I still remember. For example : if I try to visualize or write position of 'G' key on laptop keyboard, I can't. But still when I type, my fingers know its exact position. Similarly, if someone asks me to write my national anthem, I can't. But I can sing it without any mistake. Why does it happen so. Can't I remember everything in this way instead of any other memory technique? The advantage of this is quick recall.
The travel experience etc, are episodic memories. The duration of these memories mostly depend on emotion and novelty.

The typing and singing, are procedural memories. They are formed with practice. Repetition and consistency are the two most important factors for this type of memory. We do not have any conscious access to procedural memory, we simply perform what we know. 

The PIN memory can be explained with cued recall. The image of the post office is associated with the PIN, so whenever you imagine the post office the PIN number is also activated.