Will iOS 27 on iPhone and iPad communicate with Mac on Ventura?

Continuity features and requirements for Apple devices - Apple Support

Many Continuity features that involve iPhones and Macs will work with Ventura, if you meet any other requirements.

• AirPlay to Mac
• Continuity Camera: Use iPhone as a webcam for Mac
• Continuity Camera: Use iPhone or iPad to take photos and scan documents on Mac
• Continuity Sketch and Continuity Markup
• Instant Hotspot
• iPhone Cellular Calls
• Text Message Forwarding
• Universal Clipboard

Some features won't:

• iPhone Mirroring (requires Sequoia or later)
• iPhone Widgets on Mac (requires Sonoma or later)

System requirements for iCloud - Apple Support

Some "upgraded" features for iCloud sharing and Shared Albums require iOS 27, iPadOS 27, or macOS 27. A Mac running Ventura will not be able to access an "upgraded" shared album directly; to access the album, the Mac user would need to access it using a Web browser and iCloud.com.

Reference: Upgrade your shared albums - Apple Support

Otherwise it looks like Ventura supports pretty much everything, including iCloud Shared Photo Library.

It should.

If you look at the Technical Specifications for the iPhone 18 Pro, a phone that can't run anything earlier than iOS 27,

"Syncing to a Mac or PC requires:

• • macOS Catalina 10.15 or later using the Finder
  • macOS High Sierra 10.13 through macOS Mojave 10.14.6 using iTunes 12.8 or later
  • Windows 10 or later using iTunes 12.12.10 or later (free download from apple.com/itunes/download)"

I have my doubts as to how well syncing from a Mac running High Sierra, or even Mojave, will work. For a few years and versions of iOS, there have been threads in these forums where some people using High Sierra ran into issues that did not fix themselves. Issues that should not have happened, but sometimes were fixed by manually copying stuff out of Xcode (!!!) that iTunes 12.8 presumably should have been installing itself.

Now High Sierra no longer meets the (new) minimum requirements for iCloud, and at the end of January 2027, a key security certificate is set to expire. A certificate that may not have been updated in High Sierra or Mojave.

So far, there is no reason to think that Catalina and up are affected, though I don't have any plans to downgrade my current Mac from Sequoia (which it is running now) to Ventura (which it was running when I first got it) to test.

Yes, with an operating system even older than Ventura. It stands to reason that subsequent operating systems will work as well.

Will iOS27 on our phone and pad still communicate with Ventura on our Mac?

Yes, with the caveats SoC described.