Is it possible to summon entities similar to how the fill command works?

So I wish to know if it is possible to summon entities in the same way you would use the /fill command in Minecraft for example summoning a layer of arrows above the player.

I don't know if this is possible but if not is there anything similar I could do?

No you can't.

You have to define a function that records all relative positions you want the arrows to spawn. This could be done easily with code generators if you know actual programming languages.

Example: Generate a layer of arrow matrix (5x5) below the player. The function should look something like this.

execute positioned ~-2 ~-1 ~-2 run summon arrow
execute positioned ~-1 ~-1 ~-2 run summon arrow
...
execute positioned ~2 ~-1 ~2 run summon arrow

There should be 5x5 = 25 lines of code in total, each includes a relative coordinate to summon an arrow. If done correctly, this could do the job.

An in-game method to spawn entities in a certain pattern has not been developed and added in to the game. Even with the help of external generators, the process may be tedious and slow.