What is the name of this symbol that I marked in red? What is it's Unicode value?
It is called an undertie:
The undertie is used to represent linking (absence of a break) in the IPA. For example it is used to indicate liaison (e.g. /vuz‿ave/).
On computers, the character used is U+203F ‿ UNDERTIE. This is a spacing character, not to be confused with the alternative (below-letter) form of the ligature tie (a͜b U+035C ͜ COMBINING DOUBLE BREVE BELOW), which is a combining character.