Short answer:
You (almost) always put always before the verb because adverbs of frequency precede the main verb. There are always(!) exceptions that proves the rule.
Same applies to specifically. You'd usually put it precedent to the main verb but there are reasons to move it in end position which I will discuss shortly in the long answer.
I always confuse...
... they specifically target us...
... they target us specifically.
Long answer:
There are different kind of adverbs and there are different "rules" where to put them. Adverbs can take the initial, mid and end position.
Without going too deep into detail, here are some ideas on how to decide where to put the adverb. Let's start with adverbs (of manner) that modify a single word.
Broadly speaking, the adverb is preceding the word it's modifying. If the adverb modifies a verb, you place it before the verb. In the following sentence, for example, the adverb carefully modifies the verb to drive.
The man carefully drives the car.
If you want to put focus on how something is done, you can move the adverb to the end of the sentence.
The man drives the car carefully.
If the adverb modifies an adjective, you place it before the adjective. If the adverb modifies another adverb, you place it before the adverb. Those adverbs usually specify the certainty and degree of something. Here's an example for both:
The very old man drives the car extremely carefully.
The adverbs very and extremely modify the adjective old and the adverb carefully, respectively.
Adverbs of indefinite frequency (as always) are used likewise, i.e. they are in a mid position.
I always forget where to put the adverb.
Note, that an adverb is always after an auxiliary verb.
I have always been bad at remembering the position of adverbs.
Adverbs of place, time and definite frequency usually go in end position:
I learned English in school.
I saw her last week.
Again, in order to put emphasis on the adverb, you can move the adverb to the front.
The weather is still fine, but it will rain tomorrow.
The weather is still fine, but tomorrow it will rain.
Eventually, a linking adverb can take the initial position but also the position precedent to the verb. If you are unsure about this, simply take the initial position. Same applies to adverbs that have a commenting function or determine the viewpoint.
Officially, I am not allowed to tell that.
Generally speaking, learning English is fun.
However, this does not apply to me.
With this in my mind, a grammatical version of your sentence is
Why did they specifically target us?
Most natural in this situation, however, is to put the adverb at the end of the sentence as it put focus on that you care about "why us and not others".
Why did they target us specifically?
There is another alternative if you want to know the specific reason for targeting us:
Why, specifically, did they target us?
Per @Pheonix2105's answer, in some contexts there might be an implied to [do something to/with] after the word "left". But I don't see that as particularly relevant to the general case. For example...
1: Tom has 10 apples. He gives 3 to Dick. How many are left?
We neither know nor care whether Tom's 7 apples are "to eat", "to give to Harry", or whatever. All that matters is they're remaining (not given away, eaten, or otherwise removed from the original 10 apples).
In such contexts, left is simply an "adjectival" use of the past participle of the verb to leave...
2: Tom has 10 apples. Dick takes 3. How many does he leave?
Note that the last sentence there could just as well have been "How many does this leave?" - it's not really important whether we think of Dick as the "subject" who "leaves" (doesn't take) the leftover apples. If we use this, it's just a demonstrative pronoun meaning this action which has just been described.
If you're interested, see here for some interesting information about "Participle Adjectives" (such as "interested" and "interesting" as used earlier in this sentence).
Note that there are certain limitations on how adjectival left = remaining can be used...
3: Tom sold 3 of his 10 tickets. How many are left unsold? (either left or unsold could be omitted)
4: There are 7 left
5: There are 7 unsold
6: There are 7 tickets left
7: There are 7 tickets unsold
8: There are 7 unsold tickets
9: *There are 7 left tickets (idiomatically unacceptable)
...where #9 is "unacceptable" even though the basic construction is okay with alternatives which are "synonymous" in context (such as unsold, remaining). Because we're more used to adjectival left before a noun meaning "levo" (on the left, not the right side), we would normally express #9 as...
10: There are 7 leftover tickets (or left-over)
Best Answer
Only can be an adverb or an adjective. Furthermore, as an adverb it can modify a verb or an adjective.
Because of this, the position where only appears is crucial - the speaker/listener will associate it with the closest word.
So all of your sentences mean different things:
You don't do any other action besides selling apps to the young person.
You don't sell anything else but apps to the young person.
You don't sell apps to anyone except the young person.
You only sell apps to one person, and that person is the young person. This isn't too far off from the previous meaning.
Furthermore:
No one else but you sells apps to the young person
There is only one young person and you sell apps to him.