How to convert fee_estimates.dat into a human-readable format? I would like to take a look inside fee_estimates.dat to understand a bit better how the RPC command estimatesmartfee works. However, when I open the file in a text editor, the content is not readable. 

Is there a way to read the information inside fee_estimates.dat?
You need to take a look at how fee_estimates.dat is written and interpreted. It is a binary file composed of:
MIN_VERSION
CURRENT_VERSION
CURRENT_TIP
TIP_AT_LAST_START
TIP_AT_LAST_STOP
[ESTIMATES]
[STATS]

With CURRENT being &quot;when writing&quot;, i.e. when stopping bitcoind.
Here is the method writing the file, and here is the method reading this file (at startup). I think the latter can help you to parse it.