Get the outputs to/from the modal, by passing them as event emitter to the modal service and subscribe directly at the component opening the modal box.

my-modal.component.ts

import { Component, EventEmitter, Input, Output } from '@angular/core';

@Component({
	selector: 'app-my-modal',
	styleUrls: ['./my-modal.component.scss'],
	template: `
		<p>{{content}}</p>
		<button (click)="confirm()">OK</button>
	`,
})
export class MyModalComponent {
	
	@Input() content!: string;
	@Input() confirmation!: EventEmitter<boolean>;
	
	constructor() {}
	
	confirm() {
		this.confirmation.emit(true);
	}
	
}

my-whateva.component.ts

import { Component, EventEmitter } from '@angular/core';

@Component({
	selector: 'app-my-whateva',
	styleUrls: ['./my-whateva.component.scss],
	template: `
		<button (click)="doIt()">Do it!</button>
	`,
})
export class MyWhatevaComponent {
	
	constructor(
		private readonly _modalService: ModalService,
	) {}
	
	doIt() {
		let confirmation:EventEmitter<boolean> = new EventEmitter<boolean>();
		confirmation.subscribe(next => console.log(next));
		
		this._modalService.create(
			MyModalComponent,
			{content: 'Wirklich löschen?'}, // inputs
			{confirmation}, // outputs (as inputs)
		);
	}
	
}