异步更新是在Effect函数的第二参数prodEffectOptions

const setupRenderEffect: SetupRenderEffectFn = (
    instance,
    initialVNode,
    container,
    anchor,
    parentSuspense,
    isSVG,
    optimized
  ) => {
    // create reactive effect for rendering
    instance.update = effect(function componentEffect() {
      if (!instance.isMounted) {
        let vnodeHook: VNodeHook | null | undefined
        const { el, props } = initialVNode
        const { bm, m, parent } = instance

        // beforeMount hook
        if (bm) {
          invokeArrayFns(bm)
        }
        // onVnodeBeforeMount
        if ((vnodeHook = props && props.onVnodeBeforeMount)) {
          invokeVNodeHook(vnodeHook, parent, initialVNode)
        }

        // render
        const subTree = (instance.subTree = renderComponentRoot(instance))
        if (el && hydrateNode) {
          // vnode has adopted host node - perform hydration instead of mount.
          hydrateNode(
            initialVNode.el as Node,
            subTree,
            instance,
            parentSuspense
          )
        } else {
          patch(
            null,
            subTree,
            container,
            anchor,
            instance,
            parentSuspense,
            isSVG
          )
          
          initialVNode.el = subTree.el
        }
        // mounted hook
        if (m) {
          queuePostRenderEffect(m, parentSuspense)
        }
        // onVnodeMounted
        if ((vnodeHook = props && props.onVnodeMounted)) {
          queuePostRenderEffect(() => {
            invokeVNodeHook(vnodeHook!, parent, initialVNode)
          }, parentSuspense)
        }
        // activated hook for keep-alive roots.
        // #1742 activated hook must be accessed after first render
        // since the hook may be injected by a child keep-alive
        const { a } = instance
        if (
          a &&
          initialVNode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE
        ) {
          queuePostRenderEffect(a, parentSuspense)
        }
        instance.isMounted = true
      } else {      
        patch(
          prevTree,
          nextTree,
          // parent may have changed if it's in a teleport
          hostParentNode(prevTree.el!)!,
          // anchor may have changed if it's in a fragment
          getNextHostNode(prevTree),
          instance,
          parentSuspense,
          isSVG
        )
       
        next.el = nextTree.el
        if (originNext === null) {
          // self-triggered update. In case of HOC, update parent component
          // vnode el. HOC is indicated by parent instance's subTree pointing
          // to child component's vnode
          updateHOCHostEl(instance, nextTree.el)
        }
        // updated hook
        if (u) {
          queuePostRenderEffect(u, parentSuspense)
        }
        // onVnodeUpdated
        if ((vnodeHook = next.props && next.props.onVnodeUpdated)) {
          queuePostRenderEffect(() => {
            invokeVNodeHook(vnodeHook!, parent, next!, vnode)
          }, parentSuspense)
        }
      }
    }, __DEV__ ? createDevEffectOptions(instance) : prodEffectOptions)
  }

const prodEffectOptions = {
  scheduler: queueJob, //计划任务
  // #1801, #2043 component render effects should allow recursive updates
  allowRecurse: true
}

看下queueJob

export function queueJob(job: SchedulerJob) {
  if (
    (!queue.length ||
      !queue.includes(
        job,
        isFlushing && job.allowRecurse ? flushIndex + 1 : flushIndex
      )) &&
    job !== currentPreFlushParentJob
  ) {
    queue.push(job)
    queueFlush()
  }
}

function queueFlush() {
  if (!isFlushing && !isFlushPending) {
    isFlushPending = true
    currentFlushPromise = resolvedPromise.then(flushJobs)
  }
}

把异步任务放进queue然后queueFlush()冲刷，执行resolvedPromise。
vue3完全抛弃了除了promise之外的异步方案，不在支持vue2的Promise>MutationObserver>setImmediate>setTimeout其它三种异步

接着看scheduler在trigger函数中是如何执行的

export function trigger(
  target: object,
  type: TriggerOpTypes,
  key?: unknown,
  newValue?: unknown,
  oldValue?: unknown,
  oldTarget?: Map<unknown, unknown> | Set<unknown>
) {
const run = (effect: ReactiveEffect) => {
    if (effect.options.scheduler) {
      effect.options.scheduler(effect)
    } else {
      effect()
    }
  }

  effects.forEach(run)
}

可以看到run函数做了判断