WTF is a BTC Spot ETF actually???

Most media content is aimed at morons, talking about "exposure to BTC" blah blah blah... From one of the populate crypto media sites, it's just vauge.

I'm theory it would like this? If a spot ETF has 1000BTC and issues 1000 "shares", then it's share value be close to the market price of 1BTC? If the ETF issues 100,000 shares, the value goes down to 1/100th which today's about 400 bucks. Is there a clear mathematical relationship between a spot ETF BTC holds and its share price?

Or is there some way they can create an off the chain secondary market that is not backed by the asset. I hear that gold ETFs don't actually hold all the gold they say they have.

Also, if the BTC ETF is bundled with other non cyrypt securities to dilute it, wouldn't there also have to be a 100% BTC-based security on the market in the basket to calculate it's weighted impact? Can someone market BTC ETF that only has 2% of its investment in BTC securities?