How to find a deceased loved one’s executor, Will and any life insurance policies, pensions, etc.?

Contact the hospital or other medical institution (e.g. rehab facility or long-term care facility) where your fiance's father died; your fiance should call them, identify himself, and ask for help getting a death certificate. Hospitals, etc. have social workers who help family navigate the complexities of dealing with matters after a death. Either they will help him do this or they should identify to him if someone else has already come forward to take charge of matters (e.g. an executor appointed by a will); therefore, he should either be able to get the certificate or else find out who to contact.
Once he has a death certificate, he can apply to the probate court in the county in which his father lived to be appointed the personal representative or administrator of his father's estate (some states use one terms for the position, some the other)--basically, as the person who is the "executor" when there is no executor appointed by a will. Again, he should either be given that authority or told of who has already applied for and been given such authority; or alternately, while applying for this role, someone else (e.g. someone with a copy of a will) may come out to object, but at least that will bring any wills or people with other claims to the role out, where matters can be worked through.
If he is appointed administrator or executor, he will have the legal authority to enter his father's home, go through his papers and effects, contact insurance companies, banks (including for safe deposit boxes), pension plans or brokerages, etc. He will legally be able to get information and control assets. However, he should still expect to do a lot of "digging" and "leg work"--there is no central repository for this information, and you have to simply look for leads among what his father left behind. When my own father passed away, for example, I spent three days at his home, going through every drawer and file cabinet and pile of mail. Whenever I found any statement or communication from a bank, from an insurance company, etc., I called them, identified myself, and provided copies of my appointment as personal representative and the death certificate--that's how I found out about a life insurance policy, two bank accounts, and a brokerage account he'd never mentioned to me. Good luck.