Thrawn needed provisions from a developed, first world planet different from the isolated planet lost in the Unknown Regions for any sentient society. But not one directly affiliated with the Empire or the rumors he’s heard, First Order. He could procure weaponry, ships and component parts.
But what he could do is reconnect on Batuu with any Mandalorians. He found this armor made with Cortosis weave to make it stronger, unusual for most armor because it became exorbitantly expensive. Thrawn pieced it carefully together like the puzzles from his childhood, cleaned up the armor, sharpened the edges and repainted it. He had not done a build in a long, long time. What was left of his crew saw his meticulousness and applauds his efforts, despite being Mandalorian armor.
When arrived on Batuu, he wore his helmet and nodded to others, especially Mandalorians he could identify. He read a marquis of a Mandalorian cantina and stood around without a drink. Then some Mandalorian Bounty Hunters asked if he wanted to take on a short job. He agreed, despite a separate communique his crew begged him not to take the job.
His first extensive interaction with Mandalorians who thought he is Mandalorian. They did the scans. They saw the armor. They saw his weapons. Everything on Thrawn checked out as “Mandalorian” except his posture.
They completed the job, they went their separate ways. But the Mandalorian cantina did not allow him back into establishment.
He found an out of the way establishment, that does not judge, as long as you drink alone. Here he sits. Drinks a whisky, smoking a cigar sorting out what he has just done until his crew comes for him in his Mandalorian armor.