Pretend you're holding urine in, hold for about 3 seconds then release, this helps contract the muscles around the vagina
This does not tighten the vagina, rather it may reduce blood flow to it
Vagina is not rubber band
Can men do it? Yes, it helps reduce premature ejaculation by giving you control over ejaculation.
Blood flow to it is also reduced. This can lead to tears during sexual intercourse.
Putting objects also that with mess with the normal flora of the vagina can lead to infections
During childbirth what suffers the most damage are the muscles and structures surrounding the vagina,
The vagina will return to factory standards but those muscles will need some exercise to return. Hence the "looseness" after childbirth