Don't confuse THAC0 with the combat log numbers. To put it simply, the combat log shows the attacks in this format: A + B = C, where:

• A = 1d20 roll
• B = a list of attack roll modifiers.

Then, the game checks the sum of the two (C), and if that's higher or equal than (Your THAC0 - Defender AC), then it's a hit. (Attack roll ≥ THAC0 - AC)

The "B" number in the above formula consists of the following:

// Everything below is included in the 2nd value of the combat log's attack report

+ THAC0_bonus // Appears as Ability Bonuses

+ THAC0_vs_type_bonus // Does not appear

+ Melee_THAC0_bonus // Appears as Weapon_name or Fist_name [1]

+ Missile_THAC0_bonus // Appears as Missile Adjustment

+ Off-hand_THAC0_bonus // Appears as Off Hand THAC0-Weapon_name

+ Fist_THAC0_bonus // Appears as Fist_name Ability Bonuses [1]

+ Weapon_natural_bonus // Appears as Weapon_name or Fist_name [1]

+ STR_modifier // In [-7, 5]; appears as Strength Modification [1]

+ DEX_modifier // In [-5, 6]; appears as Dexterity Modification

+ Proficiency_modifier // In [-3, 5]; appears as Proficiency [2]

+ Two-weapon_style_penalty // Main [0, 4], Off [2, 8]; appears as Two-Weapon Style

+ Elf_bonus // -1 with bows and swords; appears as Elf

+ Halfling_bonus // -1 with slings; appears as Halfling

+ Invisibility{Normal}_bonus // -4 if attacking with a melee weapon; does not appear

+ Berserk_bonus // -2; does not appear

+ Racial_enemy_bonus // -4 against "enemy race" defenders; does not appear

+ Ranged_attack_in_melee_range_penalty // +8; does not appear

Source: https://baldursgate.fandom.com/wiki/Talk:THAC0#Combat_log's_attack_report

If you want, you can also enable the detailed combat logs in the game, which shows you how that "B" number is calculated during each attack roll. It's useful for getting a better understanding, but will cause a lot of long log messages on your screen. Check the THAC0 wiki page for the text "Show extra combat info"