I don't have space in my Nintendo Switch to add Pokemon Home so I downloaded it on my phone so I have a mew in Pokemon Let's Go Eevee I want to transfer it to my phone at Pokemon Home and back to Nintendo Switch on Pokemon Scarlet please tell me if it is possible and how can I do it
Only the Switch version of Pokemon HOME is able to transfer Pokemon. You will need to make space to download it --- this is most easily done by archiving game titles, which will clear most of their data but retain your save files. This is a risk-free way to save space with games you haven't played in a while. Follow Nintendo's guide if you need help with this.
Pokemon HOME (Bulbapedia)