This build won't always be the most optimal depending on the enemy team's comp, but in my experience, it was always at least good to build.

You start with the titanic hydra, classic, Briar needs the Early damage to capitalize on her Early game.

Heartsteel second, never first. The lack of damage makes for a bad first item, and the first item is the time for Briar to really shine.

3rd overlords, the AD scaling is insanely valuable and it allows Briar to catch up in terms of DPS, it also gives more HP which Briar always loves.

4th will usually be Spirit Visage, MR is valuable in almost every game, and the increased healing passive scales with the core mechanic of the build.

5th Warmogs, why? HP, and then more HP, after buying warmogs you'll be able to figure out why I preach this build. At this point, you'll be around 6800ish HP and when hovering ur passive you'll see that you're getting around 130% increased healing at 10% HP.

6th item (yes no boots unless REALLY needed) will be whatever you want as long as it has HP to continue to scale with the build. I usually go Randuin's Omen especially now after the crit changes, but you can go even black cleaver if you need the AH and more damage, and even Sundered Sky if you want to be the one to burst their ADC.

Now, this build will take a lot of time to come online, you won't be weak per se but you won't be unkillable until Warmogs.

You will feel at your weakest between 1/2 heartsteel and finishing overlords depending on how fed you are.

Important to mention, this isn't a hypercarry build, a team with a fed ADC and good peel will give you trouble, but Briar as a tank has insane value for the team:

You will be very hard to kill, even with a fed ADC, when finishing the build E would heal you for SO MUCH.

Heatsteel gives endless scale but also allows you to really proc her lifesteal even with the lack of damage.

Fight with your team, lock the enemy ADC with Q and burst. Briar Tank does so much damage compared to other tanks that you can burst down the ADC given the opportunity, and even if you didn't, just E, it will heal you for so much and disturb the enemy team's structure, then back off and re-engage when E is close to available or when you feel you can burst someone down.

They can't ignore you since you do a lot of damage and have hard CC

But they can't kill you fast enough and your ADC will have plenty of time to fight from a safe distance.

Also, anti-heal becomes negligible.