Was I running it through development_config() or local_testnet_config() ?
By default, you are running it through local_testnet_config. This default configuration can be found in node/src/command.rs in the load_spec function. See here:
fn load_spec(id: &str) -> std::result::Result<Box<dyn ChainSpec>, String> {
Ok(match id {
"dev" => Box::new(chain_spec::development_config()),
"template-rococo" => Box::new(chain_spec::local_testnet_config()),
"" | "local" => Box::new(chain_spec::local_testnet_config()),
path => Box::new(chain_spec::ChainSpec::from_json_file(std::path::PathBuf::from(path))?),
})
}
The last match arm "" | "local" is the one that runs by default (empty string). If you wanted to change which chain-spec is used (e.g. "dev"), simply provide --chain "dev" to your run command. To go one step further, play around with the build-spec command and compare the outputted files.
#generate dev spec (uses development_config)
./target/release/<node-executable> build-spec --chain dev > dev-spec.json
#generate local spec (uses local_testnet_config)
./target/release/<node-executable> build-spec --chain local > local-spec.json
Furthermore, you are correct in noting that both development_config and local_testnet_config use testnet_genesis. So, either way (in this case) the balances will come from testnet_genesis. However, you could easily modify this file to have, for example, a testnet_genesis and dev_genesis that use different balances. You could also alter which accounts have initial balances in local_testnet_config or development_config.
What determines this testnet_genesis() id argument to be 1000.into(), or 2000.into(),?
As you noted, the paraID you register on the relay chain must be placed into the id field in the chain spec.