Need to fill gaps between bedroom original floorboards

Tbh anything you do will be a short term fix due to movement in flooring. The only way you could solve this would be to change floor boards with tight joints

Gap filling expanding foam that is used to fix up windows/door frames

U could ply wood the floor with thin ply wood

Only thing out there is wood Filler but u don’t normally fit the gaps

It all depends really on the size of the gaps. If you were wanting to sand the floors and re finished them I would suggest mixed the sawdust from the sanding with wood glue and point the joints. It also depends on the integrity of the floorboards. I have known people use food filler that comes in a mastic tube ut like a previous comment said its all temporary as with any timer there is movement so would need to be regularly re done.

You can buy shims/fillets that are small slices of timber that come in various sizes. These can be glued between the boards and then once glue has hardened off use a chisel to get any protruding pieces down to floorboard level. It would be advisable to get the whole floor sanded and refinished after this to aid the blending in of new shims and old floorboard.

Expanding foam would fill the small gaps no problem but ideally change floorboards, but over time movement will usually give you slight gaps either way.