Let me just say, for the record, that no reputable manufacturer should still have been releasing 16GB phones in 2018. Least of all Samsung, whose ROMs are the largest in the business and so leave least space for the user. I know it helps hit a budget, but it causes problems like this for anyone who buys them unless they are an extremely light user.

OK, with that out of the way, there are 3 ways of moving apps to SD, but at most two of them are available on unrooted devices, sometimes just one, occasionally none. I'll describe them in the order in which I think they are most likely to be available to you. Since this has ended up longer than I intended I've highlighted the paragraph that I think is most likely to help you in red.

1) The old "android 2 style" version, where you go into Settings > Apps (or "App Manager" on a Samsung), select an app and look for a "move to SD" button. This moves part of the app (not all of it) and does not move private app data. It sounds like this is what you have now.

It was always up to the developer whether they allowed their apps to be moved this way, and not all do. That's why you cannot move some user-installed apps. In fact this option was officially deprecated in Android 4.something, and while most manufacturers added it back to many of their devices not all did, which doubtless contributes to fewer apps supporting it than used to be the case. (Samsung, ironically, didn't seem to realise this had been deprecated when they released the Galaxy S4, and spent weeks very loudly telling customers who complained about storage to move apps to SD, completely ignoring the customers who told them that the phone didn't have that option. In a move sadly typical of many corporations it was only when the mainstream press started reporting this, i.e. sales were threatened, that Samsung accepted that the option didn't exist and did something about it...).

You may be able to increase the number of apps you can move by turning on USB debugging, installing ADB on a computer, connecting the phone to the computer via USB and using the command "adb shell pm set-install-location 2". That changes the default install location to SD, and as I recall may enable moving for some apps whose developers did not enable it.

Please note: if you use a widget from an app, do not move the app to SD this way. It will look fine until you next reboot the phone, then the widget will not load.

Also note that this may not help as much as you'd like because it won't generally move app data, which for some apps are larger than the app itself.

2) Format the SD card as internal storage. This erases all data on the card, changes the filesystem to the one used by android internally, and encrypts the card. After that the card can be used as if it were part of the internal storage, so can store entire apps and their data. It can however no longer be used in other devices. It may slow the device down (fast cards are recommended), and it may reduce the lifetime of the card (card wear is increased if app data are on the card, i.e. things that are frequently written to).

Officially introduced in Android 6, but not all manufacturers added it to all of their devices. I know that most Samsungs, especially flagships, do not support this (internal storage is faster and more reliable, and my theory is that they don't want people formatting microSD cards as internal storage, particularly cheap ones, and then complaining on social media that the phone is crap when it is their card usage slowing it down). I don't know whether low-end Samsungs do though.

If you don't have this option there is nothing you can do to enable it, it's simply not part of the phone's software.

3) Rooting the phone, adding a second ext3 or ext4 partition to the SD card, and using that as additional space for apps (via an app like Link2SD is probably the simplest way). Root is required for this, so if you can't root your phone of don't want to (because some apps won't work with rooted phones) then forget this one.

This is actually the oldest solution of all, as it's what people did back before official ways of moving apps to SD existed.