An apk is an app installation package. Once that app has been installed you no longer need the apk. Does it use RAM? Only if you run it. How much? As much as the particular apk needs - that's like asking how large a file is.
If an Android phone needs RAM, it will kill an app that's in the background (one you can't see anyway), so 2GB of RAM is a bit small in mid-2018, but it shouldn't cause any problems. (When you bring that app back to the foreground, it's told to run starting at its saved state, and to you it looks as if it's been running all the time. [Some apps aen't well-written, so if you split the screen and go between the app that needs your login and password, you get the login from your password app, paste it into the app that needs it, go to the password app and get the password, go back to the app that needs it - and the login is missing. That's the developer's fault.)
But no, lack of RAM won't stop notifications from happening - you have another problem. I can't say what without having the phone to examine, but it's not a lack of RAM or a lot of apk files in storage (to totally different things) that's causing it. (But if the apps are installed, I'd copy the apk files to a PC for backup, then delete them from the phone. I have over 1,000 apps backed up as apks, but most of them aren't installed at the moment - I don't need them.)
(But a Nokia 6.1 has 3 or 4GB of RAM, not 2. If you mean a Nokia running Android 6.1, get used to posting the model number and version of Android separately. A phone running 6.1 would be an older phone [6.1 runs Android 8] and there may be differences in some questions you ask. Not in this one, though - 2GB of RAM has no effect on phone ringing or SMS notification.)
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