Buy 16 GB of DDR3L SDRAM and understand memory specs that are important today.

The F9J18UA motherboard has only one memory slot and it is inaccessible without disassembling the system, so it will be a chore to install.  A single 16 GB memory DDR3L module will be required.

Side note:  I don't know who claimed that this system will accept 16 GB of memory, but the specification sheet for the N3520 CPU says it can only handle 8 GB.  See the specs at the link below.

https://ark.intel.com/products/79049/Intel-Pentium-Processor-N3520-2M-Cache-up-to-2_42-GHz

<opinion>
In this situation I'd take it to a locally owned computer shop, have them order the memory, and have them install it.  This will preclude later accusations of "It's your memory, we only installed it."

This system is considerably underpowered to run Windows 10, and I don't think more memory will make it faster.  The N3520 has only 2MB of cache across four cores and clocks at only 2.2 GHz.  Add to that the crippling effect of a memory bus that is not bank-shared, and it's going to be slow no matter what.
</opinion>

The manufacturer's manual says there is one slot.  Pictures of the motherboard (tiny as it is) confirm that.

https://ave-electronics.com/computer-components/motherboards/oem-motherboard-cpu-fan-789089-501-7j14b0-for-hp-pavilion-11-n010dx-f9j18ua.html

Is there any way for the "system" to tell me how many slots or memory banks there are without removing any covering?

The manual says there is one.  The picture says there is one.  Therefore, there is one and it's located inside the case where it's not accessible except by taking the system apart.  Hoping for something different won't make it so.

From this list, what would be the important characteristics https://www.amazon.com/ddr3l-8gb/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Addr3l%208gb ?
Items such as PC2 vs PC3, buffered vs unbuffered, ECC vs Non-ECC, number of pins, etc come to mind

That is why you want a professional to do this for you.   They will get it right the first time, and if anything does go wrong it will be their fault, not yours.

You've got a 5400 rpm drive.
Put in an SSD.
You will see a 2 to 3x performance increase.

As for slots and memory ,go out to crucial.
The have an app that will identify what you got and what you can add.

I'd also go with kingston's memory finder https://www.kingston.com/en/memory/search/options
As you will see, it needs a more detailed model number to make sure to give you the right advice. It should be found on a sticker on your machne or in the bios. The memory finder will tell you how many slots are available and what is the maximum ram (and which modules to buy).

And by the way, it's not relatively new in my opinion. An ssd would be the best way to Speed it up. Did you even verify where the Performance Bottleneck lies?

Is there any way for the system to tell me how many slots or memory banks there are without removing any covering?

There's a few wmic command-line commands that will give you info on the PC you're using without opening it up. I use the following three to get RAM info on a particular PC.

• wmic computersystem get totalphysicalmemory
• wmic memorychip get banklabel,devicelocator,capacity
• wmic memorychip get banklabel,devicelocator,capacity

For example on one system:

wmic computersystem get totalphysicalmemory
wmic memphysical get memorydevices
wmic memorychip get banklabel,devicelocator,capacity

TotalPhysicalMemory is the total amount of RAM, MemoryDevices is the number of slots, and BankLabel shows which slots are in use and with how much RAM per slot.

Still doesn't tell how much max ram per bank is possible.

Still doesn't tell how much max ram per bank is possible.

True, I was just answering the question I quoted.

The following two commands give the maximum capacity of the PC memory array:

wmic memphysical get MaxCapacity
wmic memphysical get MaxCapacityEx

It gives the size in kilobytes. I don't think there's a difference between the two commands, except MaxCapacity is 32-bit, and MaxCapacityEx is 64-bit
https://superuser.com/questions/973417/command-wmic-memphysical-get-maxcapacity-gives-wrong-number
https://docs.microsoft.com/en-us/windows/desktop/cimwin32prov/win32-physicalmemoryarray

You can also use this command to get more info on each bank currently used:

wmic memorychip list full

https://www.itechtics.com/2-ways-check-ram-details-command-line-windows-10/

It's still good to check the hardware specs and third party sites like Kingston and Crucial to see what they think your PC is capable of.

Crucial brand memory is quality; I typically get that brand for RAM upgrades.

imostly work with Kingston Ram, but they all come with a lifelong warranty, so i guess they're all good

You don't share the model. As said, "HP Pavilion x360" is not enough, the submodel matters as some do support 2x8 GBs, while others support only 8 GBs in total. And again: the first question to answer should be: "Did you even verify where the Performance Bottleneck lies?" - maybe it's not short of RAM after all, but rather a slow hard disk. I doubt that it has an SSD and that would be the main cause for slowness at day2day usage.

i agree with the above, this model has a 5400 rpm drive :  https://support.hp.com/my-en/document/c04202035
that will benefit much more from an SSD, than from ram, but having 8 GB nowadays is nearly a must

"having 8 GB nowadays is nearly a must" - oh come on... it depends on the usage. The storage performance can be felt almost with all actions, RAM only when it is indeed insufficient and the page file is used. Standard internet usage, 10 tabs, some office documents open, some PDF files, will be nowhere next to using up these 4 GBs even with the usual background processes of AV software and some other gadgets.

I would not put 4 GBs into a new machine, no, but the author still ows us to share a view of task manager and a look at RAM usage while being slow. If RAM usage is below 80%, you can almost certainly be sure that it's rather the disk.

i also said he would benefit more from the SSD - i find you should take that into account as well

https://www.kingston.com/en/memory/search?DeviceType=3&Mfr=HEW&Line=Pavilion%20Notebook%20x360&Model=92414 there you go.
You can buy these or modules with the same specification as found at Kingston: : 8GB, DDR3 , 1600MHz , Non-ECC , CL11 ,  Unbuffered , SODIMM , 204-pin

as i said, you will have much more benefit from an SSD than from the ram upgrade, but 8 GB ram would not be bad for you, so everything is optimised

You have a machine that supports 16 GBs however, you select the solution that says, it supports 8 GBs (since the model wasn't questioned, back then).