Most Japanese use polite ending です。ます。when they are interviewed on the street. Maybe you heard する sound in the middle of the sentence. For example, “ I use apps when I study Japanese ” is 「にほんご を べんきょうする とき アプリ を つかいます」 If you want to hear natural our conversation I recommend to watch サンドウィッチマン comedy on Youtube. They are the most popular comedian in Japan and speak standard Japanese. In Japan if we don't use polite ending to older people especially we don't know them well we are regarded as bad educated.

OK, I will answer frankly and honestly. >Why is that so? Why are we, gaikokujin, expected to >use polite Japanese all the time It is not a matter of nationality but the matter of language skills (or the matter of ”空気を読む力, the skill of reading the atmosphere, what is actually going on in the conversation space) Those you watched on youtube are native speakers who can manipulate the Japanese language very well and you are not. This is the reason. >I know polite Japanese should be used when speaking to >a stranger or an older person, but I feel like most native Japanese >speakers don't really stick to that rule. It is not the rule. You (and we) can speak in whatever way you like at YOUR OWN RISK. >I've been watching "That Japanese Man Yuta" youtube channel >recently and whenever he interviews nihonjin much younger >than him, they speak to him in CASUAL japanese (for example >suru instead of shimasu) That could happen. There can be two reasons. Possible reason #1. Those young people are not descent. Possible reason #2. Those boys and girls quickly 空気を読んだ （read out the atmosphere ?) and they sensed in an instant that talking casually is WANTED by the interviewer, Yuta. I think the reason was likely to be #2. I'll tell you my story. I am a salesperson. When I meet the customer I talk 100% politely at first, but as the meeting goes, sometimes I get started to talk 99% politely and 1% casually, to make myself attractive to them. Although it is walking on thin ice, I risk doing it intentionally. A Japanese TV personality 毒蝮三太夫（Dokumamushi Sandayu） talks to elderly people so rudely like "おい、ババア、この死に損ない”, which are like "swearing" on old people. But he is very popular among old people. Is he allowed by others? No, it is just that he is playing it beautifully. https://www.youtube.com/watch?v=e1gMZU_1JJQ https://www.youtube.com/watch?v=p3_phQJWz24 (continue to the comment)