I’m sure this is answered elsewhere and asked many times but please indulge me

I’m new to the game and adore it. I am not as skilled in the world of deck building as I am in card and board game playing

I’m contemplating picking up some Investigator Expansions having seen how different investigators in the core box and impact potential outcomes

But… the bit I’m confused at is whether I can just pick up the Investigator Expansions and that’s all I need do and within the box I can easily create my own investigator decks. Or is it more involved than that? Do I need to mix cards from core and any expansion(s) I get? Or is it super straightforward? I know there are deck lists but I’m really looking for answers about investigator expansions in isolation

(Oh and when I apply xp during a campaign, if I port a character to the next campaign, do they reset or do I keep the upgrades? So thus an advantage over starting with a new character?)