mat-menu and button in different components

Well, if you want to do something like this:

<button mat-button [matMenuTriggerFor]="menu">Menu</button>
<other-component [matMenu]="menu"></other-component>
<mat-menu #menu="matMenu">
  <button mat-menu-item>Item 1</button>
  <button mat-menu-item>Item 2</button>
</mat-menu>

You can code <other-component> like this:

import {Component,Input} from '@angular/core';
import {MatMenu} from '@angular/material/menu';

@Component({
  selector: 'other-component',
  template: `
    This button is in another component:
    <button [matMenuTriggerFor]="matMenu">Click here to open menu</button>
  `,
})
export class OtherComponent {
  @Input() matMenu: MatMenu;
}

You can see the above example working at this stackblitz demo.

---

Another approach

Another approach is (I think this is what you want): your trigger button is inside the parent (but outside the child) and the menu itself is defined inside the child component.

Parent component:

<button mat-button [matMenuTriggerFor]="childComponentMenu?.menu">
    Menu in other component
</button>
<child-component></child-component>

export class ParentComponent {
  @ViewChild(ChildComponent) childComponentMenu: ChildComponent;
}

Child Component:

@Component({
  selector: 'child-component',
  template: `
    <mat-menu>
      <button mat-menu-item>Item 1 (inside other component)</button>
      <button mat-menu-item>Item 2 (inside other component)</button>
    </mat-menu>
  `,
})
export class ChildComponent {
  @ViewChild(MatMenu, {static: true}) menu: MatMenu;
}

---

Yet Another approach

Another approach, similar to the above one, but using template reference variables (notice the exportAs in the decorator of the child component):

Parent component:

<button mat-button [matMenuTriggerFor]="x.menu">
    Menu in other component
</button>
<child-component #x="menuInOtherComponent"></child-component>

export class ParentComponent {
}

Child Component:

@Component({
  selector: 'child-component',
  template: `
    <mat-menu>
      <button mat-menu-item>Item 1 (inside other component)</button>
      <button mat-menu-item>Item 2 (inside other component)</button>
    </mat-menu>
  `,
  exportAs: 'menuInOtherComponent',
})
export class ChildComponent {
  @ViewChild(MatMenu, {static: true}) menu: MatMenu;
}

Stackblitz demo

Here's another solution using ng-content. That's my preferred way to go.

my-custom-menu-component html:

<div [matMenuTriggerFor]="menu">
  <ng-content></ng-content>
</div>

<mat-menu #menu="matMenu">
  menu content
</mat-menu>

parent-component html:

<my-custom-menu-component>
  <button>click me</button>
<my-custom-menu-component>