The record player: the whole thing.
The deck: those parts which turn the record.
The platter: the big round thing upon which you put a record.
The stylus: the tiny piece which engages with the record groove.
The cantilever: the small tube which carries the stylus.
The cartridge: the thing from which the cantilever protrudes.
The arm, or tonearm: the beam which carries the cartridge & swings across the record.
Decks are separated into two types. Both have an electric motor which drives the platter. The motor can be rigidly attached to the platter, in which case the deck is called a direct drive deck. Or the motor drives a belt which drives the platter, in which case the deck is called a belt drive deck. One type is not inherently better than the other; it depends on the quality of the individual design.
In both types the platter revolves on a bearing. If you cannot turn the platter by hand then it sounds as if the lubrication of the bearing has dried, sticking together the parts of the bearing. If you are mechanically adept then disassemble the deck & renew the bearing (recommended) or relubricate it. In either case, when you have the bearing out take it to a bearing shop & see if they have a replacement. If so get one. If not, ask them what lubricant they suggest. In the latter case, to renew the lubricant soak the bearing in degreaser & then in isopropyl alcohol (IPA) or acetone. Do not move the bearing parts relative to each other while the bearing is clean. Apply new lubricant. Reassemble the deck.
If you are not mechanically adept then get the help of a mechanical fitter.
This bearing is relatively large, depending upon the wight of the platter. The arm uses a number of much smaller bearings & I recommend the same procedure. Be warned: this is a delicate assembly & must be handled carefully. It might not be possible to disassemble it, in which case it will probably be scrap.
Regarding the belt, in the case of a belt drive deck, get a new one: www.gbaudio.co.uk/data/belts.htm
It sounds as if the manufacturer of your player is long gone, so you are on your own. Good luck.