It often depends on the procedure & length of same. Both methods may involve both injections and intubation (which may administer gas/es or just oxygen/air to keep the airways open & working).

Around here, for spays/neutering, they usually use an injected dissociative anesthetic followed with IV and/or a gas mixture....

Yup, sometimes ya gotta keep an eye on the little guys. OTOH, it can be a bit humorous/entertaining watching/helping them come out of it. :p

Doc