Several files: source

Keep shared functions in another file and load them with source.

Runs in Zerethon IDE · Bash 5.2 Official Bash documentation ↗

main.sh

source ./lib/strings.sh

shout "hello"
repeat ab 3

lib/strings.sh

shout() {
  echo "${1^^}!"
}

repeat() {
  local out="" i
  for (( i = 0; i < $2; i++ )); do
    out+="$1"
  done
  echo "$out"
}

Output

HELLO!
ababab

Two files: main.sh (the entry) and lib/strings.sh with the functions.

source file (or . file) runs the file in the current script, so its functions and variables stay defined.
Paths are relative to the working folder, which is the project root.
Running bash other.sh instead starts a separate script; nothing it defines comes back.

Official references