Longest common prefix, binary search problem

class Solution {
public:
string longestCommonPrefix(vector& strs) {
if (!strs.size()) {
return "";
}
int minLength = min_element(strs.begin(), strs.end(), [](const string& s, const string& t) {return s.size() < t.size();})->size();
int low = 0, high = minLength;
while (low < high) {
int mid = (high - low + 1) / 2 + low;
if (isCommonPrefix(strs, mid)) {
low = mid;
}
else {
high = mid - 1;
}
}
return strs[0].substr(0, low);
}

bool isCommonPrefix(const vector<string>& strs, int length) {    string str0 = strs[0].substr(0, length);    int count = strs.size();    for (int i = 1; i < count; ++i) {        string str = strs[i];        for (int j = 0; j < length; ++j) {            if (str0[j] != str[j]) {                return false;            }        }    }    return true;}

};
ask ：int minLength = min_element(strs.begin(), strs.end(), [](const string& s, const string& t) {return s.size() < t.size();})->size(); I can't understand this code , I hope to be a responsible boss , Help me analyze them one by one , I don't understand the grammar of this [](const string& s, const string& t)
ask ：bool isCommonPrefix The end of this function return true, I'm so confused , Isn't it that every call to this function will return true Do you , If not why
A responsible big man