The configuration file referenced by man 5 sway-input is the sway configuration file in `~/.config/sway/config` or `~/.sway/config` or other allowed locations. You can find the default configuration in `/etc/sway/config`.
To set your chosen layout(s) it should be enough to add something similar to this in the configuration:
input type:keyboard {
xkb_layout fr,us # two layouts
xkb_variant oss,intl # their respective variants
xkb_options grp:sclk_toggle # toggle between layouts with the scroll-lock button
xkb_numlock enabled # enable numlock when logging in
}
You can find the codes for all layouts, variants and toggle keys listed in `man xkeyboard-config`.