At the time of writing, which is 10th edition 40k, Catachans are units within Codex: Astra Militarum. In 8th and 9th edition, Catachan was a designation you could apply to any Astra Militarum army.
There are 3 datasheets that are specifically Catachan: Catachan Command Squad, Catachan Jungle Fighters and Catachan Heavy Weapons Team. You don’t have to include any of these, e.g. if you want a Catachan tank company.
There are some other datasheets that are named for other regiments (Cadian and Krieg), but the majority of datasheets are not specific to any regiment.
The printed codex is not up-to-date, as there have been FAQ and Errata applied to it, but if you buy it, it includes a code which allows you to see up-to-date rules in the 40k App. Up-to-date points for all factions are available from warhammer-community.com, as are the errata documents.
Various sites online host the rules without permission from GW.