盤子 like many nouns with only one character (比如: 椅子，桌子，房子，兒子/儿子，帽子，鬍子，杯子，叉子，筷子) is followed by 子, called the noun suffix, when the noun appears by itself.

I think it helps to resolve ambiguity inherent in speaking (none of those would be ambiguous when written).

Often when those same nouns appear in compounds the 子 is dropped.

When you put 上 in front of a noun, it often makes it a verb，i.e. 上床, 上船。When you want to use the word to mean on something, it goes after the noun.

In modern Chinese, the proposition is always after the object. And here 上 means the proposition "on".

在 has two meanings. If it is followed by a verb, it is the continuous marker. If it is followed by a prepositional phrase, it is the verb "locate".

So:

在桌子上 = locate (be) on the table

在上桌子 = getting upon the table (上, as a verb, means "get upon")

In fact, in English it's "on the table", and "on" means 上，but here you can't think in a English way, in Chinese, prepositions are always put afterwards, so it's 在桌子上，you can use this kind of pattern"在……上", which means "on... ".

In Chinese, the preposition is after the noun

上 in that position would be more like a verb, but is ungrammatical in this case anyway. As a location complement, always after the object!