Inherited Sharing vs No Sharing declaration

When inherited sharing is used, it means that it will inherit the sharing settings of the caller class.

• If a class with without sharing is calling the method of class with inherited sharing, then it will run in without sharing mode.
• If a class with with sharing is calling the method of class with inherited sharing, then it will run in with sharing mode.
• If a class with omitted sharing (i.e. no sharing is mentioned) is calling the method of class with inherited sharing, then it will run in without sharing mode as the default sharing mode of the omitted sharing is to run in without sharing mode (This is because the calling class is established as a without sharing context). A class declared as inherited sharing runs as without sharing only when explicitly called from an already established without sharing context.
• If a class with with sharing is calling a method of class with omitted sharing which in turn calls a method of class with inherited sharing, then it will run in with sharing mode, as the calling class is in with sharing mode and omitted class is established as with sharing context.
• If a class with without sharing is calling a method of class with omitted sharing which in turn calls a method of class with inherited sharing, then it will run in without sharing mode, as the calling class is in without sharing mode and the omitted class is established as without sharing context.
• If the class is used as the entry point to an Apex transaction, an omitted sharing declaration runs as without sharing.
• If the class is used as the entry point to an Apex transaction, inherited sharing ensures that the default is to run as with sharing.

Excerpt from the salesforce documentation, Using the with sharing, without sharing, and inherited sharing Keywords

There is a distinct difference between an Apex class that is marked with inherited sharing and one with an omitted sharing declaration. If the class is used as the entry point to an Apex transaction, an omitted sharing declaration runs as without sharing. However, inherited sharing ensures that the default is to run as with sharing. A class declared as inherited sharing runs as without sharing only when explicitly called from an already established without sharing context.

Hope answers for both your questions are covered in the above explanation.

If you can refer salesforce docs then you observed the difference between inherited sharing and no sharing. Excerpt from salesforce docs:

There is a distinct difference between an Apex class that is marked with inherited sharing and one with an omitted sharing declaration. If the class is used as the entry point to an Apex transaction, an omitted sharing declaration runs as without sharing. However, inherited sharing ensures that the default is to run as with sharing. A class declared as inherited sharing runs as without sharing only when explicitly called from an already established without sharing context.

Please refer below link for more details:

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_sharing.htm