I want to find the Bitcoin I purchased in the past by paying 100,000 won.

I remember back then that I made the purchase by bank transfer or phone small payment. Is this the right way to purchase it?
I think I formatted and installed Windows on the disk about 4 to 5 times. Is recovery possible? (Currently, the wallet.dat file cannot be found)
I connected the drive to another desktop, installed Bitcoin QT, and synchronized it over three days. Currently, blk files in newly installed qt are confirmed from blk04197 to blk04209. The blk file starts from blk00000.dat. Why is it created like this? If it is strange, is it correct that there should be files from blk00000.dat to blk04196.dat? If so, can you find that file?
Thank you for your reply. I am a person who does not know much about computers.