O carregador foi colocado na tomada da cozinha, mas não funciona bem.

foi colocado is a passive voice construction: ser + past participle. It means was placed/was put and focuses on the charger, not on who did it.
If you wanted the active, you could say:

• (Eles) colocaram o carregador na tomada da cozinha, mas não funciona bem. = They put the charger...

It’s ser (to be) + participle:

• foi = past (preterite) of ser for ele (it)
• colocado = past participle of colocar (to place/put)

So: foi colocado = was placed.

The participle agrees with the subject o carregador (masculine singular), so it must be colocado.
Examples of agreement:

• O carregador foi colocado (masc. sg.)
• A tomada foi colocada (fem. sg.)
• Os carregadores foram colocados (masc. pl.)
• As tomadas foram colocadas (fem. pl.)

na is a contraction of em + a:

• em = in/on/at
• a = the (feminine singular)

tomada is feminine (a tomada), so:

• em a tomada → na tomada

Meaning: in the outlet / into the socket / plugged into the outlet (natural translation depends on context).

da is a contraction of de + a:

• de = of/from
• a = the (feminine singular)

So da cozinha = of the kitchen → the kitchen’s (as in the kitchen outlet).
tomada de cozinha would sound more like an outlet for kitchen use (more generic/category-like), not specifically the outlet in the kitchen.

In Portuguese, it’s common (and usually recommended) to use a comma before mas when it connects two clauses, especially when the subject changes or there’s a clear contrast:

• Clause 1: O carregador foi colocado...
• Clause 2: (ele) não funciona bem.

Because the sentence is describing a current result/state: it was plugged in (past event), but it doesn’t work well (now). Portuguese often uses the present for a situation that’s true at the moment of speaking.
If you meant it didn’t work at that time (past-only), you could say:

• ..., mas não funcionou bem. = but it didn’t work well.

bem means well, so não funciona bem = doesn’t work well / doesn’t work properly.
You can omit it:

• ..., mas não funciona. = ..., but it doesn’t work. (stronger/more absolute)

A very common informal alternative in Brazil is:

• ..., mas não funciona direito. = ..., but it doesn’t work properly.

Yes, informally you may hear:

• O carregador foi plugado na tomada... (from plugar, a common borrowed verb)

But foi colocado na tomada is more neutral and broadly acceptable. Another natural option is:

• O carregador foi colocado na tomada / O carregador foi ligado na tomada (also common in Brazil depending on region).