Hi and welcome
Guinea pigs regulatetheir body temperature via the blood flow through their ears; that is why they have an individually shaped and sized bald area behind the ears.
As a rule of thumb, if things are symmetrical on a guinea pig (i.e. have the same shape and feel on both sides of the body), then it is highly likely normal.
Here are our very helpful guides for learning about the body quirks in guinea pigs to work out what is normal for your own piggies, so you can then spot any changes in your life-long weekly weigh-ins and health checks, which are part of your important health monitoring regime so you can spot developing problems early on and address them when they can be treated medically instead of when they have turned into s desperate fight for life.
Guinea pig body quirks - What is normal and what not?
Weight - Monitoring and Management
How To Pick Up And Weigh Your Guinea Pigs Safely
Early Signs Of Illness
These guides are all part of our very comprehensive practical information resource for new owners. You may want to bookmark the link, browse (we have some rather fascinating things in there together with some that can really make a difference in quality and length of life), read and re-read at need. You will get different things out of some the guides depening on the level of experience.
Here is the link:
Getting Started - New Owners' Most Helpful Guides