The phrase "similar internals" means that two things have the same or very similar inner workings or components, even though they may look different on the outside. Full definition
The benchmark scores are quite lesser than what you would see in devices with similar internal specs, but in actual, the performance is good for regular use. (phoneradar.com)
And often times there are similar internal positions with a description already in place. (careerdigital.com)
Both phones have very similar internal specs, with a Snapdragon 835, 4 GB of RAM and 64 GB of storage. (androidcentral.com)