hiiii

Sn1 and sn2 reactions are nucleophilic substitution reactions in both the positive charged central species attract and forms a week electrostatic bonding.
If the bonding is very temporary and lasts very soon thats sn1 but if bonding is somewhat strong and both the species come together for a time together then all it's sn2 reaction.

Replace positive charge with negative and negative with positive will convert it into electrophilic substitution reaction only charge is the difference in most cases.
But some cases are very different and are subject of research.
That's all basic           !!!!!