I got this reference from the video (linked) where he's talking about the differences between Mandarin and Japanese, mainly involving 漢字 which is present in both languages but there are actual differences (besides simplified or traditional characters) associated with either printed text and handwritten formats, kind of like this:
I mean, it's literally the same word in both languages but they're different if you look closely. No wonder why people mistake Japanese for CHINESE! It's infuritating when you know what Kanji actually looks like and how it's pronounced. The issue is that characters for the most part inherit same unicode between Japanese & Mandarin.
Mainly talking about word processors and how Kanji / Hanzi is encoded onto computers, what ends up happening is that they overlap depending on the font used (as Japanese has it's own font set while Mandarin uses a completely different one). Look at it like this, which language is the "A" looking letter from (shown below):
Both use the same font, but have the stroke on top facing different directions indicating they are from different languages. However if one doesn't pay close attention to the letters present within a word, they can face confusion over which language it belongs to. That's the kind of crap happens when discussing Kanji or Hanzi.
For example, the unicode for 軍 is U+8ECD which is present in both Japanese and Mandarin. Basically, the same as to why the letter "Í" (U+CD) appears in multiple languages like Spanish, Portuguese, Icelandic, Hungarian, Czech all inherit unicode U+CD so you may get mixed results in one of those languages.
It's not really clear what your question is. Certainly the pronunciation of the same letter is different between different languages using the same (or nearly the same) alphabet. In some cases, the same pronunciation is indicated by different letters: Swedish and Norwegian use ö and ø, respectively, both to indicate the sound [ø].
The modern letter j is descended from the Latin letter i. Latin itself did not distinguish them. This is a somewhat similar situation to the various languages using Han characters, where while the characters are broadly speaking the same, there are some variations which carry meaning.
These differences don't suffer from the problems of Han unification within Unicode. Because the alphabets are much smaller than the set of Chinese characters, there is no problem to replicate every single small difference. This means that even letters which are clearly "the same", like the Greek letter "omicron" and the Latin letter named (in English) "oh" actually have different codepoints.
One last semi-related point: handwriting varies quite widely between different countries using the Latin alphabet, meaning that while on the internet letters may seem to be identical, they can have wildly different shapes!