Help to bypass play store??

I have an app on my phone that acts as a vault for private information. It is perfectly invisible on my phone until you enter the play store then it's obvious that I have the app installed....
How do you by pass the app store or hide the app being installed from the app store but keep it on my phone? I've tried downloading .apk but that doesn't work.
Not rooted running android 8.1 Thank you!

James, thank you for the reply. I have already attempted to download the app through chrome through their website directly to bypass the play store but afterward it still shows up in play store as being installed and is listed in my installed apps.. Could I disable the play store download the app then reactivate the store? Doesn't seem like it would work.. Or is there a way to rewrite some of the code, add a line or change a line to confuse the play store into thinking it's not installed ? I can't leave the app store disabled because what I am going for is complete invisibility. No red flags nothing to cause hostility. I need it so well hidden because certain people have access to my phone that I can not take away without causing problems. Im at air right now but I appreciate all answers and feedback.