What are the most common tech stacks the contractors use ? How can someone get into contracting ?

Depends on the area. Banks and finance use a lot of contractors. Java/Spring reigns supreme, but there are gigs for C#, C++ etc

There is a lot of contract work in Frontend/Backend and understandably it uses most popular stacks: Javascript/React/Ruby/Rails/Python/Django/PHP etc

Same as employees.

There is nothing special about contractors, it's just a tradeoff fiscal benefits / employer's flexibility.

Look at remote jobs boards and filter by contract or keywords lime "agency"

How would working for competing apps work ? Would drivers be considered employees of both Uber and Lyft ? Do they have to log a certain amount of miles driven in order to qualify as an employee ? I'm interested in seeing how this all works out.