Thank you, I found how to do it, it's really easy!
What I didn't get is that I had to make a directory in src/main/resources and name it data.minecraft.loot_tables.blocks and place my JSON file in it. By just naming the package "data.minecraft", it automatically overrides Minecraft's loot tables!
To replace the loot table with your custom item, that's how you need to write the JSON file:
And name the file oak_log if you want to override the Oak Log loot table for example.