What are the key differences between being a "deemed employee" or a genuine contractor for IR35 purposes?

The key differences between being a "deemed employee" and a genuine "Outside IR35" contractor for IR35 purposes are:

Substitution: An employee cannot provide a substitute to carry out their work for them. At the same time, a contractor has the unfettered right to send an equally qualified replacement to complete the job.

Control: An employee is subject to a significant level of control over their work. At the same time, a contractor is fully autonomous and free to determine how they provide their services.

Mutuality of obligation: Employees must make themselves available for work and are expected to be offered work or paid otherwise. A contractor is only ever paid for work done, and the client is not obligated to provide them with work, nor is the contractor obligated to accept the work offered.

Other Factors: A whole host of other factors can be considered, which help discern whether the contract is one of employment or a commercial contract. These can include (but are not limited to) financial risk, provision of equipment, indemnities, IP ownership, etc.

Considering employment status is highly fact sensitive and not a simple tick-box exercise like ticking off the items on a list and counting them up. It's a multi-factorial assessment that requires looking at the entire picture and taking an evaluative approach.