first time buying pc part

im new to pc stuff and got few questions about ram. i got school pc like a month ago with 4gb ram and i want to get 8gb ram but got few questions about it.

i've seen some fail ram install videos before (when ram breaks) and want to know if it's hard to install it and let somebody else do it or if i should do it by myself.

i use this pc for mostly game developing and gaming and right now i think ram is most important thing that i need to get but im not sure how it'll work. will it like combine and be 12gb or will some apps run on one ram and some on another.

and i also wanna know how much of a difference there will be in multitasking and gaming/

gpu: intel uhd 610
cpu: intel pentium g5420
i know its very weak pc but it works