prepositions +ing

[allthewayanime]

Up until now I've been learning that prepositions require the ing form; now I've came across these 2 sentences and I don't understand why the infinitive is correct after 'but'.
In my opinion because the verb before "but," "do," is in a simple (non-continuous) tense therefore it requires the simple tense . In the second example, the ING-form is used because the verb before "but" is in a continuous tense, therefore it requires the ing-form

I would like to know if my understanding is right and if this pattern is only used with 'but' or can it be used with other prepositions too?

'They do nothing but complain.'
'They are doing nothing but bumming around.'.

[Preceptor]

'But' is a conjunction, meaning that it will connect two similar grammatical structures.

"They do nothing but complain." This is actually a compound sentence that could be written out completely as, "They do nothing but they complain." This is to say that all they do is complain.

"They are doing nothing but bumming around." This is a similar situation. It is a shortened way of saying, "They are doing nothing but they are bumming around."

So 'but' is not a preposition.

[allthewayanime]

To sum it up the prepositions require the ing form and the conjunctions can have both the bare infinitive and the ing form?

[5jj]

I don't agree with this: "They do nothing but complain." This is actually a compound sentence that could be written out completely as, "They do nothing but they complain."

In the original, 'but' has a similar meaning to 'except'. It cannot be be expanded to a sentence with a contrasting 'but'. 'They do nothing but they complain' makes sense to me only if 'do nothing' implies 'do nothing productive/creative/etc', which conveys a different meaning from the original. That (the original) means, as Preceptor said, This is to say that all they do is complain.