Any XF user who is marked as a staff member will not have nofollow applied to any of their links, including signatures.
Marking a user as a staff member is actually separate to their moderator or administrator state so you may need to go and edit your user record in the Admin CP and check this box:
After that, the external links in your own signature will simply be noopener but that's an important security feature which is applied to all external links.
Is the setting 'nofollow' not harmfull for spidering the entire site. I mean when I am f.e. on this page https://xenforo.com/community/forums/xenforo-suggestions.18/ I see al lot of 'nofollow' on internal links, which means that the google spider shall not follow that link. For external links I understand the 'nofollow' attribute but for internal links I don't.
Same question for the 'nofollow' attribute on the 'what's new'section.