Unfortunately, you cannot check if a player is holding it, but you can check if a player has it on Bedrock Edition.

You can use this function to run it. Replace stick, stone, and ~ ~ ~ with what paintbrush item, what block you want, and the coordinates or where a player is looking, respectively.

Always active repeating command block

/clear <player> stick -1 0

This checks if a player has the paintbrush. Chain conditional:

/tag <player> add brush

Chain conditionals:

/execute @a[tag=brush] setblock ~ ~ ~ stone

/tag  <player> remove brush

Replace <player> with the player you want to target. I hope this helps.

It looks like you’re thinking that addons for bedrock work like mods in java. They don’t. Most of the things you see in Minecraft are not possible in the JSON/JavaScript Bedrock API. You can’t make a bow, sorry. The items in bedrock are limited to the following behaviors: blocks, seeds, ordinary items, foods, potions, or a combo of food and potions (like a gapple), and maybe some other random thing I forgot.