"Reading aloud" is reading out loud and not in your head. "Reading loudly" is when you read aloud, but you do it very loudly, like shouting.

Hi Junnan. "Read aloud" = read so that other people can hear you (read and speak at the same time). For example, in a language class a teacher might say "Junnan, please read the first paragraph of the text aloud". You can also say "Read out loud" which has the same meaning as "read aloud". Loudly = noisily/in a way that produces a lot of noise. For example: "He was talking very loudly in the library and everyone was annoyed". So, if someone is "reading loudly" they are reading in a noisy way (although this is quite strange haha). "Read loud" isn't correct.

Read aloud = read out loud = say the words as you're reading. Read loud is technically wrong, although people do say it. It should be read loudly because read is a verb, so you need an adverb. Either way, it means read aloud... but with more volume...like a teacher talking to a student in class...read the paragraph aloud, but as there's a lot of noise coming from outside, read loudly.