I am not sure of the specifics for leaving things to animals, however I would be surprised if you did not have to set up some trust or something and make the cats the responsibility of that, possibly also making your boyfriend a trustee or something and thus having your house to live in or whatever. I don't think it is a valid will to say "I, DinohScene, of sound mind and my own volition, as my last will and testament do leave the sum total of wealth and possessions in my estate to be shared equally between my cats Pissy Knickers, Boots, and Dave. DinohScene. 30th of Feb [some year]". I could be wrong but most things I see of "rich idiot leaves penthouse to goldfish" are usually a trust or some business arrangement, or at very least a condition on the sale of a property that reads "for the rest of their life and then to charity" or something.
On debts and "negative things" then you can't just foist them on other people, even your family, and they mostly die with you. There are things you can do like multi generational mortgages but that is not as much a debt as a possibly nice thing (if you are living somewhere where such things are a viable option for some then you have just given them a headstart, or the option to sell the equity). If you hear of such things then it is usually because the dead person had something and someone else wants it (say a family home or something rare or sentimental value) or a stake in a business that the remaining part of the might not want to go out to open sale, however as their estate has bills to pay then they end up paying for that first, possibly with difficulty if they lack liquid assets, to get it/keep it. If uncle fuckwit dies with a lot of debt and names me in the will then I am free to say keep his collection of soup cans and leave me out of it entirely. It can get far far more complex, surprising that what with it being an area of law and accounting, and there are all sorts of checks and balances, traps, taxes and tricky things that people do, but a fundamental aspect is you are perfectly free to walk away from debts that are not yours (do note that in the case of certain types of insurance or being a guarantor for a debt).
I guess you could try to convince them that millions are secretly buried under the floor of your beach hut and then get them to pay the debts to get at it but that would be active effort.