4/ I used to have the linkedin premium recruiter plan, and the most important filters are: 
- languages, tools, technologies 
- year of graduation (can tell me overall years of work experience) 
- location (tax reasons) 
- come from similar cos

so optimize these on your linkedin profile if you are open to web2 jobs

5/ Pipeline management - What happens once I submit my resume to a job posting? 
- Due to the large amount of scam applications, recruiters will often add dealbreaker questions. It is very important that you answer these questions. Once you answer yes/no, you're sorted into a digital "fit" or "not a fit" pile.

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