What must come out, and why
Anything with a sealed battery is a hazard at cremation temperatures. Pacemakers and implantable defibrillators are the common ones; some neurostimulators and drug pumps fall in the same category. These are removed before the cremation, and the authorizing agent is normally asked to confirm in writing whether such a device is present.
This is not a formality on the form. A pacemaker in a retort damages equipment and endangers the operator, which is why the question is asked directly and why an honest answer matters even if the family is unsure.
What survives
Hip and knee replacements, plates, screws, surgical pins and dental gold survive the process. They are separated magnetically or by hand afterwards, and they are not part of what is returned to the family — cremated remains are bone, not metal.
Most facilities recycle the recovered metal through specialist firms, and many direct the proceeds to charity. Ask what your provider does with it; you are entitled to know, and practices vary.
Jewellery
Remove it beforehand. Rings, watches and chains will not survive, and gold that does survive is separated with the other metal rather than returned. If a family wants a ring cremated with the person as a deliberate choice, say so explicitly — but understand it will not come back.
This is worth a specific conversation before transfer, not after. Once the person is in care, retrieving an item is a request rather than a simple matter.
What ashes actually are
What is returned is processed bone fragment, pale grey to white, typically three to seven pounds for an adult. It is not ash in the fireplace sense, and its weight tracks skeletal mass rather than body weight — which surprises families who expect a small person to leave little behind.