i heard if you use "many/much" alone in a positive sentence, it could sound kinda stiff.
and i also heard they both are usually used in a negative and interrogative sentence
so could you please tell me whether the sentences below sound awkward or not? thanks a lot ^^

1.
there're many people on the street

2.
i believe they have many meanings

3.
many people asked him to sing a song

4.
there're many mosquitos flying around in here

5.
many people sleep at night

6.
i picked up much trash around where we were

7.
there're many implications in that sentence

I can only answer for my dialect, but in sentences 1, 4, 5, and 6 I would say "a lot of" instead.

In sentences 2 and 3 I would probably say "a lot of" but I also might say "many" or "several."

In sentence 7 I think that "many" actually has a slightly different connotation than "a lot of," so I would say "many" there.

But yes, overall, I agree with the things you've heard. I think it's the kind of thing where if you say it once or twice, no one will think it's weird since we do say it sometimes in positive sentences, but if you're saying it frequently it can start to seem a little odd or stilted.