The hold rewards are funded by the withdrawal fee that others pay when withdrawing their funds.

When the another party pays the fee in UNI, that UNI is redistributed to all other network participants in proportion to the percentage of Total Network Value they hold.

Finally, the UNI reward that you receive is converted to the tokens that you hold. If your portfolio is 60% OMG and 40% BAT, and you receive a reward of 10 UNI, you will earn 6 UNI worth of OMG and 4 UNI worth of BAT