How do you use Google API getRequestHeaders() to get an OAuth2 access token?

This new function directly gives you an 'Headers' object:

{ Authorization: 'Bearer xxxxxxxxx' }

So you can happend it directly use it as your header object:

const authHeaders = await this.auth.getRequestHeaders();
yourfetchlibrary.get('https://......', {
            headers: authHeaders,
        })

Or extract the Authorization part:

const authHeaders = await this.auth.getRequestHeaders();
yourfetchlibrary.get('https://......', {
            headers: {
                'Authorization': authHeaders.Authorization,
                'Content-Type': 'application/json',
            },
        })

const accessToken=oAuth2Client.getAccessToken()

This worked for me