The only way they could ever take it from you:

1) you gave up your seed willingly, probably under coercion

2) you had it custodied with a company whose assets they seized (much like leaving your fiat with a bank)

How they might know what Bitcoin is yours:

1) you bought it from another party who had your information attached to the transaction (such as an exchange)

2) you paid for a product with your name attached to it

That's the basics without getting too convoluted