Upgrading RAM in 14Q-cs0009tu, Product: 5DZ92PA#ACJ OS: Win 10 (64-Bit) Intel Core i3-7020U@ 2.30GHz, warranty over RAM 4GB 2133 MHz (slots 1of 2) [closed]

I have a hand me down 14Q-cs0009tu (working fine).
I want to upgrade its RAM because I don’t require much, but my children and grandkids do.

I’m confused because of the advise I have received. Some people say I should upgrade to only what I have {4GB}.
Others say I can upgrade to 8GB.
Others say I can upgrade up to 16GB.
Some even suggest 16×2=32GB

One post also says Modern RAM runs best when it’s installed in matched memory module pairs – that’s the “Double Data Rate” part of DDR4. If just a single memory module is installed, or mismatched modules are used, then the data rate will be half of the specification. For example, DDR4-3200 would run at just 1,600MHz.

I don't understand it:
Does that mean I don’t have a choice and have to install a 4GB module running at 2133 MHz in the other slot?
I don’t have the timings or the latency rate for the present so you can understand my problem.

Do I/Can I Add 8GB in the other slot?
OR 16GB?
OR am I bound by 4GB in the 1st slot?
What if I give up on him and have 8×2, or even 16×2?
What’s the max?

AND Keeping the the volume and speed in consideration (including CL) what’s the best combination?

Please don’t shoot for the moon as I’m a veteran I can spend only so much.

Per https://www.kingston.com/us/memory/search/model/98446/hp-compaq-14-series-notebook-14q-cs0xxx-intel-pentium-celeron (which is a reasonably authorative source as they guarantee compatibility using it), the system has a single memory slot, so you will need to replace the RAM you have to increase it.

The maximum RAM you can apparently add is 8 GIGS (ie DDR4 2666MHz Non-ECC Unbuffered SODIMM CL19 1RX8 1.2V 260-pin 8Gbit - possibly using Kingston part KCP426SS8/8)