The majority of applications are a one-time purchase, perhaps with access provided for future updates.
There are some apps which are considered a “subscription,” but they are always explicitly labeled as such…and IMHO seem like a scam. I feel like any subscriptions should be separate from the app-purchase.
Once you buy an app, you “own” it. It’s a one-time payment, and you’ll always have access to it on that Google account. If you buy a new phone or a second tablet, and use the same Google account on it, you can install the app on those. (There may be a limit on how many devices an app can be installed at the same time, but it’s more than 2 or 3.)
Generally you get free updates too. Though sometimes the developer will release a major new version of an app as a new app. You can still keep using the old version, but if you want the new version, you have to buy it.
Often these are listed as ‘free’ in the store, but after you download and install them you quickly find out that the app won’t really do much of anything unless you buy a subscription to it. At least that’s how it seems with a lot of iTunes ‘subscription’ apps. The ones from reputable developers make it clear in the descriptions that it requires a subscription to be useful, less reputable ones are little more than a ‘commercial’ for the subscription service rather than an actual app.