Neetu,
I am able to understand the concepts like below:
Heterogeneous Portfolio:
Assume this portfolio is having 10 policies, each poicy covers a building in an area.
Het Portfolio = Policy1 + Policy2 + .....+ Policy10
Policy1 is issued to provide coverage for a building in Area1
Policy2 is issued to provide coverage for a building in Area2
......
......
Policy10 is issued to provide coverage for a building in Area10
Here the policies provides the coverage for different buildings located in different Areas.
Homogeneous Portfolio:
Assume this portfolio is having 10 policies, each poicy covers a building in an area.
Homo Portfolio = Policy1 + Policy2 + .....+ Policy10
Policy1 is issued to provide coverage for a building in Area1
Policy2 is issued to provide coverage for a building in Area1
......
......
Policy10 is issued to provide coverage for a building in Area1
Here the policies provides the coverage for different buildings located in only one Area.
Assume that if a catastrophe happens in Area1, extreme damage occurs and all the policies issued in that area can raise claims. By knowing the number of claims for policy1, we can easily guess the number of claims for other policy too since all the buildings are located in the same area and all are subjected to the same type loss.
Based on this we can say that, the number of claims for each policy under a homogeneous portfolio is similar.
Coming to heterogeneos portfolio, since the policies covers buildings situated at different areas, the number of claims raised by any two policies may not be the same. Thats why they are heterogeous. For more explanation on this go thru the eg

Motor insurance portfolio) given in the material.
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Coming to the calc of variance for the whole portfolio:
For this we need to understand the difference between the
'Two random variables are independent' and
'Two random variables are Conditionally Independent'
E(X1X2) = E(X1) E(X2) - Two Random variables (X1,X2) are independent.
E(X1X2/Y) = E(X1/Y)E(X2/Y) - Two Random variables (x1,x2) are conditionally independent.
To get the clear idea on this please have a look at the first eg: given in the credibility theory.
Just as discussed above in the homogeneous portfolio, the number of claims for each policy is almost similar, lets say it is 'lamda'.
Based on the credibility theory eg, we can say that aggregate claims 'Si'/lamda for i = 1 to n are independent & identically distributed since the claim distributions F(x) are also similar.
In the case of homo si s are just independent i.e, not conditionally independent. So overall, here the si s are iid.
The variance formula for 'Sum of aggregate cliams where the aggregate claims are independent' is different from the 'Sum of aggregate claims where the aggregate claims are conditionally independent'
Hetero:
Var[Sigma si] = n var(si) since si s are iid, var(s1) = var(s2) = ...= var(sn)
Homo:
Var(S) = E(Var(s/lamda)) + var(E(s/lamad)), since aggregate claim depends on lamda. In the summation we need to apply this for every si and known that si/lamda are iids.