I've been struggling my way through the first basic lesson for a week but consulting alphabets elsewhere on line, the vowels are listed without the circles. Are the circles place holders? Do you ever see them once you see actual Korean written?
Circles make no sound but are written. They are place holders. For example: 아 and ㅏ makes the same sound, but 아 is correct. 안녕하세요. 아빠. 아저씨. 아, 좋아. 아기. 엄마. These are examples. ㅏ makes the aah sound. 아 also makes the same sound.
Despite being an alphabet, the way you use the Hangul (the Korean alphabet) is like a syllabary, which is a fancy "alphabet" made of syllables, like the Japanese hiragana and katakana.
That means that when you are actually writing sentences in Korean you have to group those characters into syllables, with at least 1 consonant and 1 vowel, and the consonant always comes first. So when you need to write a syllable that begins with a vowel sound, you use a special consonant, the ㅇ (ieung), that has no sound at the beginning of a syllable (at the end it sounds like the English ng).
Basically, you will always see them in actual written Korean. You will never see vowels and consonants by themselves unless you're reading texting slangs and abbreviations, like ㅎㅎㅎ and ㅋㅋㅋ for laughter, ㄱㅅ for thanks and so on.
Here, this might help. I don't know, though. It might not be what you are looking for. https://tiny.cards/decks/672592cc-f2cd-47f7-a85d-b9ff5b5a7b25