I've never even seen, let alone fought Thargoids before, and my experience of the content so far is...
Spoilers;
Everyone seems to be getting interdicted as they enter HIP 22460, this was my first meeting of them, a wing of 3, and they shut down my ship, before honking at me and flying off.
Upon arrival in system, your UI will glitch out and any Anti-Xeno gear you have will rapidly decay to 0 and break. This includes FSD boosters, so make sure you have the range to leave again.
There are wreckage fields about, and broken megaships. I went to Bright Wotsit to try and scan the data points, but whilst there a Thargoid dropped in, and disabled my ship entirely, then opened fire. But I've been a good boy with our insect bros! I drifted into the megaship hull, then eventually got control back, and saw my hull was already down to 60%. As I've not fought before and wasn't sure what to do, and had no AX weapons working anyway, I just legged it. By the time I leapt out of system and back to my own Carrier nearby for repairs, I was down to 32% hull; I am not sure what kind of Thargoid attacked me and if it was particularly deadly, but I don't think it was anything special.
The system chat was especially toxic, with people expressing complaints and then being trolled by people who have mistaken making a purchase with having a personality. There were rumours of a "Thargoid Mothership" but it could just also be trolling. Wait for verified information before you go in, but my suspicion is one clever way to timegate the content so anyone can see it is to break everyones AX stuff and prevent them fighting the Thargoids at all.
I asked if anyone was trying with the standard weapons, but got no answer. I presume it's still largely impossible with out AX?
Basically, dont worry so far folks, I'm not seeing anything that HAS to be seen immediately after launch. However the cut scene that launches when you first log in doesn't seem, as far as I can tell, to appear in your Codex or elsewhere, so don't skip it, or youtube it if you miss it in game.