C-SPC to mark each file individually or mark all with
M-a, then press
RET to open all those files. I wrote a Helm guide here that covers Helm basics and most of default commands. Also check my helm-projectile. It has the multifile opening right at the beginning.
Aside from the answer above, the question was updated with the question to open multiple files using
helm-projectile-switch-project. To use
helm-projectile-switch-project to switch to another project and open multiple files, either
helm-projectile-find-file must be set to
(setq projectile-completion-system 'helm
After that, you can open files in any project (including current projejct) without ever leaving your current working project. This is also explained in Enter project portal: helm-projectile-switch-project, C-c p p in my guide.