So as l've understood the only way to get PVP is to get to an instanced zone called Cyrodiil? If this is true this is a major let down

As it would seem, you have not yet been there. You should check it out, You wont be disappointed.

Cyrodiil is an open world province in the centre of the continent, you join a campaign and the description says each campaign lasts 90 days - these campaigns are the instances.

The zone is an entire world unto itself. But you won't be able to gank lowbies, sorry.

Cyrodiil is absolutely massive and has pleny of PvE content like repeatable quests in several hubs and public dungeons scattered around the map so there will deffinently be some small scale pvp and ganking going on.

Would anyone go to Cyrodiil for pve tho? I get that
It's big and open etc but it's still closed out from the other world. I'd want players to attack me while questing or gank me. That's the element of surprise that's fun. Going into Cyro Is like a big BG with some pve content it seems like. Is there anything that forces you to go there? Like a main stort or something?

Yeah, only stealh [censored] gankers care about open world PvP.

PvP is by consent - you go to the zone in the full understanding that it's a PvP zone. Not everyone wants to be PvPed at any given moment in any given location.

ESO has the best pvp system nowdays. It's a huge continent with pve quests and many options of warfare tactis. Indeed, players don't need to go there is they don't want, but 90% will.