Fetching list of fields for an object

I was looking for APIs to list all fields of an object. I found the /fields APIs for the following three objects-

Account: /assets/account/fields?depth={depth}&count={count}&page={page}&search={search}&orderBy={orderBy}&lastUpdatedAt={lastUpdatedAt}&extensions={extensions}

Contact: /assets/contact/fields?depth={depth}&count={count}&page={page}&search={search}&orderBy={orderBy}&lastUpdatedAt={lastUpdatedAt}&extensions={extensions}

Campaign: /assets/campaign/fields?depth={depth}&count={count}&page={page}&search={search}&orderBy={orderBy}&lastUpdatedAt={lastUpdatedAt}&extensions={extensions}

The above three objects are the objects which support the list of field values in their respective request bodies.

I needed to know the following:

i) Is the functionality of fetching all fields of an object supported for only those objects which support creation of custom fields? (for eg. Account - AccountField, Contact - ContactField etc)

ii) Is there any other way we can fetch a list of all the fields given an object?

iii) For the responses of /fields APIs mentioned above, does isStandard=false implies that the field is a custom field created by the user?

We need to take decisions in our development approaches accordingly.