Overview
minecraft:loot is a block component: it goes in the components object of a custom block. Picks the loot table that decides what drops when the block is broken. Give the path inside your behavior pack, like "loot_tables/blocks/my_block.json".
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:loot": {}
}