You can browse the catalogue without one. To download assets, even free ones, you need a free 3DTexel account. Click Connect to 3DTexel in the panel and log in from your browser; you never type a password in Blender.

The handmade CC0 assets are free. Assets from the Community library (made on 3dtexel.com by the community) and the Auto-Rig (30 credits) use 3D Texel credits. Paid actions always show the price and your balance first, and assets you already own never cost credits again.

Yes. Handmade assets are CC0 (public domain): use them for anything, no attribution needed. Community library assets come with the 3D Texel License. Each asset shows its licence in the panel.

Yes if its bones use Mixamo names (with or without the mixamorig: prefix) or Unreal mannequin names. Characters rigged with the 3DTexel Auto-Rig use the Mixamo skeleton, so motions apply directly. Other rigs get the motion with its own skeleton.

Blender 4.2 to 5.2. It installs as an Extension (Get Extensions > Install from Disk) and works in Cycles and EEVEE.