Custom User Interface

How do I create custom user interfaces with a Minecraft Addon? All I see in defaultreourcepack/ui is these .json files:

account_banned_screen.json
account_transfer_error_screen.json
achievement_screen.json
add_external_server_screen.json
adhoc_inprogess_screen.json
adhoc_screen.json
anvil_screen.json
anvil_screen_pocket.json
authentication_modals.json
authentication_screen.json
auto_save_info_screen.json
beacon_screen.json
beacon_screen_pocket.json
blast_furnace_screen.json
book_screen.json
braze_screen.json
brewing_stand_screen.json
brewing_stand_screen_pocket.json
bundle_purchase_warning_screen.json
cartography_screen.json
cartography_screen_pocket.json
chalkboard_screen.json
chat_screen.json
chat_settings_menu_screen.json
chest_screen.json
choose_realm_screen.json
coin_purchase_screen.json
command_block_screen.json
comment_screen.json
confirm_msa_unlink_screen.json
content_log.json
content_log_history_screen.json
convert_purchases_to_xbl_screen.json
create_world_upsell_screen.json
credits_screen.json
custom_templates_screen.json
day_one_experience_intro_screen.json
day_one_experience_screen.json
death_screen.json
debug_screen.json
dev_console_screen.json
disconnect_screen.json
edu_discovery_dialog.json
emote_wheel_screen.json
enchanting_screen.json
enchanting_screen_pocket.json
expanded_skin_pack_screen.json
feed_common.json
feed_screen.json
file_upload_screen.json
furnace_screen.json
furnace_screen_pocket.json
gamepad_disconnected.json
gamepad_layout_screen.json
game_tip_screen.json
global_pause_screen.json
grindstone_screen.json
grindstone_screen_pocket.json
hdr_calibration_screen.json
holographic_postrender_screen.json
horse_screen.json
horse_screen_pocket.json
host_options_screen.json
how_to_play_common.json
how_to_play_screen.json
hud_screen.json
immersive_reader.json
inventory_screen.json
inventory_screen_pocket.json
invite_screen.json
in_bed_screen.json
item_detail_description_screen.json
jigsaw_editor_screen.json
late_join_pregame_screen.json
library_modal_screen.json
local_world_picker_screen.json
loom_screen.json
loom_screen_pocket.json
manage_feed_screen.json
manifest_validation_screen.json
mob_effect_screen.json
non_xbl_user_management_screen.json
npc_interact_screen.json
online_safety_screen.json
pack_settings_screen.json
patch_notes_screen.json
pause_screen.json
pdp_screen.json
pdp_screenshots_section.json
perf_turtle.json
permissions_screen.json
persona_common.json
persona_screen.json
play_screen.json
pocket_containers.json
popup_dialog.json
portfolio_screen.json
post_rating_screen.json
profile_card.json
profile_screen.json
progress_screen.json
rating_prompt.json
realmsPlus_screen.json
realmsPlus_sections
realmsplus_upgrade_notice_screen.json
realms_common.json
realms_create.json
realms_pending_invitations.json
realms_plus_ended_screen.json
realms_settings_screen.json
realms_whitelist.json
redstone_screen.json
resource_packs_screen.json
safe_zone_screen.json
scoreboards.json
screenshot_picker_screen.json
screenshot_screen.json
server_form.json
settings_screen.json
settings_sections
sign_screen.json
simple_inprogress_screen.json
skin_pack_purchase_screen.json
skin_picker_screen.json
smithing_table_screen.json
smithing_table_screen_pocket.json
smoker_screen.json
start_screen.json
stonecutter_screen.json
stonecutter_screen_pocket.json
storage_management.json
storage_management_popup.json
store_common.json
store_data_driven_screen.json
store_filter_menu_screen.json
store_gift_promotion_screen.json
store_inventory_screen.json
store_item_list_screen.json
store_sales_item_list_screen.json
store_search_screen.json
store_sort_menu_screen.json
structure_editor_screen.json
sync_iaps_to_xbl_screen.json
tabbed_upsell_screen.json
test_anims_screen.json
third_party_store_screen.json
toast_screen.json
token_faq_screen.json
trade_2_screen.json
trade_2_screen_pocket.json
trade_screen.json
trade_screen_pocket.json
trial_upsell_screen.json
ugc_viewer_screen.json
ui_art_assets_common.json
ui_common.json
ui_common_classic.json
ui_edu_common.json
ui_holo_cursor.json
ui_purchase_common.json
ui_template_buttons.json
ui_template_dialogs.json
ui_template_tabs.json
ui_template_toggles.json
update_version.json
win10_trial_conversion_screen.json
world_conversion_complete_screen.json
world_conversion_error_report_screen.json
world_panorama_screen.json
world_templates_screen.json
xbl_console_signin.json
xbl_console_signin_succeeded.json
xbl_first_launch.json
xbl_friend_finder.json
xbl_optional_signin.json
xbl_report_user.json
_global_variables.json
_ui_defs.json

What do they do? How can I change the UI of a screen with these files?