It's just the normal text messaging on my phone.

oh ok ty

Yes I do. My husband was using it so I downloaded it to see what it's about and it's all about encryption. Messages can only be seen on the devices from where the message was sent or received. The contacts saved can usually be found in Contacts in the Google account though.
Usually whatever device is being used it will have it's own messaging app. I have a Samsung Galaxy so I have the Samsung messaging app that came as a default app. Google Messaging usually has to be downloaded from the Google Play Store and then set as the default app in the phone settings to be able to use it. You can still usually see the messages in the original messaging app, but won't be able to send messages with it unless in the phone settings you change it back to it as the default messaging app.
Messages sent from or received from Google Messages aren't always encrypted, say if the other person isn't using a messaging app that supports encryption like the Google app does.

I questioned this , I asked to see her phone she refused she said she don't have it , but it shows up on her google Activity ( she dont know i know i saw it ) i smoothed it over by saying its suppose to be a new " private messaging" .. someone said its part of your text messaging so? I checked mine and its not there at all, this just started last month ...so ? I am sure a coworker told her, I also found 2 new phone no# on the account
That sucks tho some spouses are sneaky ...its like wtf god forbit Morrgin if you or I did it , I check her tablet .. but the App has to be on the phone but thanks for in put makes me wonder if shes still talking to her ex coworker boyfriend

You can also open Google Play, click on their profile pic and it will show a menu (android), from there select Manage Apps and Devices, select the tab that says Manage and you get a list of all the apps they've been using or are currently using. I found Telegram app on my husband's phone today

So now she's using messages by Google and Whatapp , never ends 61 yr will get a surprise

My husband's mistress along with his family pushed me out, while my husband was battling end stage cancer. Point of my story, I kinda had suspicions a couple years before while he was well. However, I loved him and wanted our happy ever after. He died with his mistress and not me (the one who never cheated and was completely devoted to love and our wedding vows). BABE, BE TRUE TO YOURSELF FIRST AND FOREMOST.

thank you for your input Melt2020