I need to find the location of the code which deals with UTXO, and then implement FIFO on UTXO storage. I aint able to find the location of UTXO code and also where it is stored.
What you want is the code for Florincoin's wallet's coin selection. This is found in the
CreateTransaction function. Within
CreateTransaction, the wallet also calls
SelectCoins and that calls
SelectCoinsMinConf. The coins are actually selected with
CreateTransaction are all involved with coin selection.
Also, because Florincoin's wallet is based on Bitcoin Core, it does not use FIFO for coin selection. Rather it uses Bitcoin Core's special algorithm for selecting coins which is difficult to understand (it tries to solve the knapsack problem by selecting exactly the amount you want to spend, but then it does some weird things to handle fees and change).