Help me understand some lore !!

Hi, I apologize if these are dumb questions, I'm just trying to get a better understand of FFXIV's lore. I am currently at the start of ShB and the lore is confusing to me

What I'm confused about:

• What is the source? What are shards, exactly? Where does ARR - SB take place? Are we in a specific shard?
• To that end, do calamities only happen in "our" continuity, whatever that is?
• I keep hearing mention of the "eight and final" umbral era. What's significant about the next calamity?
• More generally regarding calamities, does anything physically "happen" to the shards or those dwelling within them when a rejoining happens?
• What happened with Hydaelyn and Zodiark

I'll probably think of more questions if I get some responses, but I really appreciate any effort to help explain these things to me. Also as much as possible do try to keep the responses to pre ShB as i do not want to spoil ShB for myself :D

EDIT: Thank you everyone for helping me understand the lore i can do the msq now with less confusion !