Hi 

Install  web_responsive  module by OCA, This Module allows users to select Chatter Position from Preference,

https://apps.odoo.com/apps/modules/14.0/web_responsive/

Hope it Helps,

Kiran K

For my future self or any future odoo backend developer. Here is what I did to make chatter position to bottom permanently. Please note that I'm using odoo 16

                    .o_form_view.o_xxl_form_view {

                        flex-flow: column nowrap !important;

                    }

                    .o_form_view.o_xxl_form_view .o_form_view_container {

                        width: 100%;

                    }

                    .o_FormRenderer_chatterContainer.o-aside {

                        max-width: 100%;

                        width: 100%;

                        height: 100%;

                    }

                    .o_form_view.o_xxl_form_view .o_form_sheet_bg > .o_form_sheet {

                        max-width: 100%;

                    }