Most in the series come with 4Gb of RAM soldered in, so you get another 16Gb stick[1].
Regarding the SSD, typically you get a laptop with one installed, but you can add in the m.2 ssd slot[2].
Then you buy the battery pack [3].
You should be good to go from there. I've seen people replace the screen, trackpad, and keyboard as well, but those are all I typically replace.
[1] https://www.amazon.com/Crucial-Single-PC4-19200-Unbuffered-2...
[2] https://www.amazon.com/Samsung-960-PRO-Internal-MZ-V6P512BW/...
[3] https://www.amazon.com/Lenovo-Battery-0c52862-Factory-Sealed...