Алексей

let initToIndex = arr1.reduce((acc, item, idx) => {
	acc[item[0].init] = idx;
	return acc;
}, {});

arr2.flat().forEach(x => {
	let idx = initToIndex[x.init];
	if (!initToIndex.hasOwnProperty(x.init)) {
		idx = arr1.push([]) - 1;
		initToIndex[x.init] = idx;
	};
	
	arr1[idx].push(x);
	
});

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