When you are creating your resume, you don’t need to put every random job you’ve ever had. What companies do is they look at your jobs on the resume, and at most call the employer and ask them if you worked for them and how you did at the job.
There is no way for a non government employee to know if you worked other jobs. Keep off any jobs that you worked at for less than 2 years and use every skill you learned as a skill for your resume.
Nothing hurts your resume more than having 3 or 4 jobs in a span of 2 years because it shows you are unreliable.
This is highly industry-dependent. When I was working in IT and systems admin, I had a lot of contract/temp jobs that were still valuable experiences. My resume after finishing university would have been blank if I left those 3-6 month contracts off because that’s how you get your foot in the door in a lot of fields.
Oh for those really short contracts I just make dates up. I can never remember when I started and finished. I just look at the calendar and make sure I started on a Monday and finished on a Friday and it’s roughly within the right time range. Never been called out on it.
I put month and year for start and end dates and keep my CV updated regularly.