It seems you are trying to get all build and release definitions. If this is the case, you should refer to the following apis:
Gets a list of definitions:
GET https://dev.azure.com/{organization}/{project}/_apis/build/definitions?api-version=6.0
With optional parameters:
GET https://dev.azure.com/{organization}/{project}/_apis/build/definitions?name={name}&repositoryId={repositoryId}&repositoryType={repositoryType}&queryOrder={queryOrder}&$top={$top}&continuationToken={continuationToken}&minMetricsTime={minMetricsTime}&definitionIds={definitionIds}&path={path}&builtAfter={builtAfter}¬BuiltAfter={notBuiltAfter}&includeAllProperties={includeAllProperties}&includeLatestBuilds={includeLatestBuilds}&taskIdFilter={taskIdFilter}&processType={processType}&yamlFilename={yamlFilename}&api-version=6.0
Get a list of release definitions:
GET https://vsrm.dev.azure.com/{organization}/{project}/_apis/release/definitions?api-version=6.0
With optional parameters:
GET https://vsrm.dev.azure.com/{organization}/{project}/_apis/release/definitions?searchText={searchText}&$expand={$expand}&artifactType={artifactType}&artifactSourceId={artifactSourceId}&$top={$top}&continuationToken={continuationToken}&queryOrder={queryOrder}&path={path}&isExactNameMatch={isExactNameMatch}&tagFilter={tagFilter}&propertyFilters={propertyFilters}&definitionIdFilter={definitionIdFilter}&isDeleted={isDeleted}&searchTextContainsFolderName={searchTextContainsFolderName}&api-version=6.0