Would like to ask ruling on if one realizes after any wajib ghusl that there has been some stain or dirt inside the nail which is not so very clear or not even alot, would it invalidate my ghusl or I should not worry about it? My nails are medium size which I usually cut time to time. What is the ruling on external part of body? What about eyelashes, if you see a tiny bit of mascara on your eyelashes after ghusl can I just ignore that and continue salaat or my ghusl becomes unvoid?

During Ghusl it’s not obligatory to wash those body parts which not included as outer surface of one’s body for example it’s not obligatory to wash inside your nose or ears or eyes same way if nails are not as long as one could say that it has separated from the skin of the finger then one has to wash them during Ghusl as normal and no need to try to reach water inside the joining area where the skin of the tip of the finger and nails joints together.
But if nails are long enough and if there any obstacles seen under that enlarged nail then it must be removed.
External part of the body must be free from any type of obstacle and oily substance which can be barrier for water to reach up to the skin. As far as eyelashes are concerned, it’s very clear that it’s the part of outer body so it must be free from any obstacle.