How does typescript check valid when type assertions happend in typescript [duplicate]

I am learning type assertion concept in typescript. After Reading tutorial offical documentation, older version and some others, I have been confused about how typescript evaluates type assertions as valid.

the following example will make my problem more clear:

I have some type in the code:

type typA = {
    name: string;
};

type typB = {
    name: string;
    age: number;
};

type typC = {
    isGraduated: boolean;
};

type typD = {
    address: string;
    age: number;
};

type typEmpty = {};

then i created some variables that have the above types:

let a: typA = {
    name: "John",
};

let b: typB = {
    name: "Alex",
    age: 10,
};

The problem happen when i run the code below:

a as typEmpty;           // (1) Ok
b as typEmpty;           // (2) Ok

a as typB;               // (3) Ok
b as typA;               // (4) Ok

b as typC;               // (5) error: ts(2352) Conversion of type '...' to type '...' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
b as typD;               // (6) error: ts(2352) Conversion of type '...' to type '...' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
1 as string              // (7) error: ts(2352) Conversion of type '...' to type '...' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.

b as never as typC;      // (8)  suprisingly, Ok
b as unknown as typC;    // (9)  suprisingly, Ok
b as any as typC;        // (10) suprisingly, Ok
1 as never as string     // (11) suprisingly, Ok

I have wonder that Why (8), (9), (10) and (11) are Ok while (5),(6), (7) make error?

I have searched a lot before pushing question in here, but i have not found reasonable answers yet.

Thanks for considering my stupid question.