Kitaristi soitti suosikkini aivan lopussa.

A simple breakdown is:

• Kitaristi = subject
• soitti = verb
• suosikkini = object
• aivan lopussa = adverbial phrase telling when in the sequence

So the structure is basically:

subject + verb + object + time/position phrase

That is a very natural Finnish sentence pattern.

Finnish does not have articles like English a/an and the.

So kitaristi can mean:

• a guitarist
• the guitarist

Context tells you which one is meant. In this sentence, English would often use the guitarist, but Finnish just says kitaristi.

Soitti is the past tense of soittaa.

• soittaa = to play, to call
• soitti = played, called

Here it clearly means played, because the subject is a guitarist and the object is something musical.

Grammar-wise:

• the past tense marker is -i-
• soitti is 3rd person singular, so it means he/she played

Finnish does not need a separate word for he or she here.

Suosikki means favorite or favourite one.

The ending -ni is a possessive suffix meaning my.

So:

• suosikki = favorite
• suosikkini = my favorite

In other words, Finnish often puts possession directly onto the noun instead of using a separate word like my.

Suosikkini is enough by itself.

The ending -ni already means my, so minun is not required.

You could also say:

• minun suosikkini

But that is more explicit or emphatic. In many normal sentences, just suosikkini is perfectly natural.

This is about the object case.

In this sentence, the guitarist played a complete, whole thing: your favorite song/piece. That usually gives a total object in Finnish.

So:

• suosikkini = total object here
• suosikkiani = partitive object

A partitive form like suosikkiani would suggest something less complete, such as:

• only part of it was played
• the action was ongoing
• the object is being treated as less bounded

So suosikkini fits the idea of playing the whole favorite song/piece.

Yes, suosikki is a noun here.

It literally means favorite / favourite one. In context, it probably refers to something like:

• my favorite song
• my favorite piece
• my favorite tune

Finnish, like English, often allows that understood noun to stay unspoken if the context makes it clear. So suosikkini can naturally mean my favorite one.

Aivan lopussa means right at the end or at the very end.

The word lopussa is the inessive form of loppu, which literally means something like in the end.

So:

• loppu = end
• lopussa = in/at the end

Finnish often uses location cases for time or position within a sequence, not just physical place.

Compare:

• lopussa = at the end
• loppuun = to the end
• lopuksi = finally / in the end / as a conclusion

Aivan adds emphasis.

Here it means something like:

• right
• quite
• absolutely

In this sentence, the natural English sense is right at the end or at the very end.

So:

• lopussa = at the end
• aivan lopussa = right at the very end

Yes. Finnish word order is fairly flexible.

This sentence has a natural, straightforward order:

• Kitaristi soitti suosikkini aivan lopussa.

But you could move things around for emphasis. For example:

• Aivan lopussa kitaristi soitti suosikkini.

That version puts more focus on when it happened.

Even though Finnish allows movement, the original order is very normal and easy to understand.