Do you know what "blockchain library" means? I don't get their wallet core product at all.
Suppose Trust Wallet is a car, the Wallet Core library is only an engine. Without chassis, tires, etc. In order to drive it you need to implement and integrate it on your own. In the same way, you can't do anything with only an engine.
Electrum wallet, on the other hand, already gives all of the parts so the user only needs to build and then use it. The thing about Trust Wallet, their tires, chassis, and any other parts are unknown besides the engine. So that is why I did not categorize it as open-source software.
Is it part of their multi-blockchain support capability? Which other codes are missing from this (apart from the UI for example)?
Yes.
What the library mainly does are:
Wallet management
-Creating a new multi-coin wallet
-Importing a multi-coin wallet
Address derivation (receiving)
-Generating the default address for a coin
-Generating an address using a custom derivation path (expert)
Transaction signing (e.g. for sending)
Everything other than that is a closed source.