Let $f(x)=(x + a)(x + b)$ where $a,b>0$. For any reals $x_1,x_2,\ldots ,x_n\geqslant 0$ satisfying $x_1+x_2+\ldots +x_n =1$, find the maximum of $F=\sum\limits_{1 \leqslant i < j \leqslant n} {\min \left\{ {f({x_i}),f({x_j})} \right\}} $.
1. **Define the function and the problem constraints:**
   Let \( f(x) = (x + a)(x + b) \) where \( a, b > 0 \). We need to find the maximum of 
   \[
   F = \sum_{1 \leq i < j \leq n} \min \{ f(x_i), f(x_j) \}
   \]
   for \( x_1, x_2, \ldots, x_n \geq 0 \) such that \( x_1 + x_2 + \ldots + x_n = 1 \).

2. **State the lemma:**
   For any \( n \geq 2 \) and \( T > 0 \), if \( x_1, x_2, \ldots, x_n \geq 0 \) satisfy \( x_1 + x_2 + \ldots + x_n = T \), then 
   \[
   F_n(x_1, x_2, \ldots, x_n) = \sum_{1 \leq i < j \leq n} \min \{ f(x_i), f(x_j) \}
   \]
   is maximized if and only if \( x_1 = x_2 = \ldots = x_n \).

3. **Proof of the lemma:**
   Suppose \( x_1 \leq x_2 \leq \ldots \leq x_n \). Then \( f(x_1) \leq f(x_2) \leq \ldots \leq f(x_n) \). Therefore,
   \[
   F_n(x_1, x_2, \ldots, x_n) = \sum_{i=1}^{n-1} (n-i) f(x_i).
   \]

4. **Base case \( n = 2 \):**
   \[
   F(x_1, x_2) = f(x_1)
   \]
   Since \( 0 \leq x_1 \leq \frac{T}{2} \), we have
   \[
   F_2(x_1, x_2) = f(x_1) \leq f\left( \frac{T}{2} \right).
   \]
   Thus, \( F_2(x_1, x_2) \) is maximized if and only if \( x_1 = \frac{T}{2} = x_2 \).

5. **Inductive step:**
   Assume the lemma is true for some \( k \geq 2 \). Now suppose \( x_1 + x_2 + \ldots + x_{k+1} = T \). Then,
   \[
   F_{k+1}(x_1, x_2, \ldots, x_{k+1}) = k f(x_1) + F_k(x_2, x_3, \ldots, x_{k+1}).
   \]
   If \( F_{k+1}(x_1, x_2, \ldots, x_{k+1}) \) is maximized, then \( F_k(x_2, x_3, \ldots, x_{k+1}) \) is maximized under \( x_2 + x_3 + \ldots + x_{k+1} = T - x_1 \). By the induction hypothesis,
   \[
   x_2 = x_3 = \ldots = x_{k+1} = \frac{T - x_1}{k}.
   \]
   Therefore,
   \[
   F_{k+1}(x_1, x_2, \ldots, x_{k+1}) = k f(x_1) + \frac{k(k-1)}{2} f\left( \frac{T - x_1}{k} \right) = g(x_1),
   \]
   where \( x_1 \in \left[ 0, \frac{T}{k+1} \right] \). The function \( g(x_1) \) is quadratic in \( x_1 \) and can be written as
   \[
   g(x_1) = \left( k + \frac{k-1}{2k} \right) x_1^2 + Ax + B.
   \]
   Since \( k + \frac{k-1}{2k} > 0 \), the maximum of \( g(x_1) \) occurs at the endpoints:
   \[
   \max \{ g(0), g\left( \frac{T}{k+1} \right) \} = g\left( \frac{T}{k+1} \right).
   \]
   Thus, \( x_1 = x_2 = \ldots = x_{k+1} = \frac{T}{k+1} \). This completes the induction.

6. **Conclusion:**
   By the lemma, the maximum of \( F \) is
   \[
   \frac{n(n-1)}{2} f\left( \frac{1}{n} \right) = \frac{(n-1)}{2n} \left( na + 1 \right) \left( nb + 1 \right).
   \]

The final answer is \(\boxed{\frac{(n-1)}{2n} \left( na + 1 \right) \left( nb + 1 \right)}\).