Its ى , which is similar to ي . In the middle of a sentence, so that's why you dont fully recognize it. To find it on your phone keyboard, hold down ى until ئ appears. It then performs the same as ي in writing (حائط/ حايط)
Hamza or ء is a guttural stop, like the stop in co-op. It is a bit special because it can stand alone in a sentence or accompany any long vowel ( ء ؤ ئ أ ). This word is a bit unique (to me in this course) because it has a hamza in the middle on a vowel, but in essence its no different than ء
It is a hamza ء . It is not an Alif ا nor is it a ي nor a ى . The ى is just the carrier of the hamza.
There is a whole set of rules what letters have to be used as carriers for hamza depending on long and short vowels or absence of vowels. The whole set of rules lead too far at this point in the course, unfortunately, but you can start collecting: after a long a and before a short i in the middle of a word, the carrier is a ى (without dots).
This has given me a lot of trouble, as well. The way I resolved it was by reading the Arabic keyboard file on my computer (it was at /usr/share/X11/xkb/symbols/ara ; I'm a Linux user). I could not find any documentation to help with this; I have arabic fonts, etc., installed, but there seem to be some characters (such as superscript alif) which are not in obvious positions on the keyboard. So, for example, on my keyboard, the superscript alif is at AltGr + n (that's relative to the English keyboard). But it would be really really helpful, since duolingo seems to insist on characters that aren't in obvious places on Arabic keyboards, if we could be pointed towards a document that explained them. ARE YOU LISTENING, DUOLINGO?
Yashanna1: on my computer, I typed out the three rows of Arabic letters into a document, spaced to look just like the actual keyboard, printed it out, and stuck the printout to the wall in front of me. So now, if I place my hands on the keyboard as for touch-typing, I'm learning to find the keys I need without looking. Good luck!
The site forvo.com has an example pronounciation: https://forvo.com/word/%D8%A7%D9%84%D8%AD%D8%A7%D8%A6%D8%B7/#ar