Short answer
The official FAQ supports solo play, cooperative play and cross-platform multiplayer. You can choose how to enter a raid instead of being forced into one fixed squad format.
What is not stated here
The exact current squad cap, revive rules and party restrictions should be checked in the launch client before being presented as universal facts. Search suggestions for “how to revive teammate” reveal interest, but they are not a source for the mechanic.
Practical starting choice
Solo is useful for learning the basic loop. Co-op is useful when you want shared callouts and a second opinion on risk. Both choices still require the same extraction discipline.