Really depends on the contests rules. Take for example the following contests that are active right now and how they are joined.
Main page of consts:
growdiaries.com/giveaways
growdiaries.com/giveaways/DotM-August-2022
growdiaries.com/giveaways/NotM-August-2022
growdiaries.com/giveaways/GotM-August-2022
growdiaries.com/giveaways/PotM-August-2022
these are the normal monthly contests that happens all the time.
they require you to upload diaries or photos in order to join, some of them must be from a specific breeder like MOTM, but the rest are any breeder.
growdiaries.com/giveaways/Fast-Buds-Meme-contest-august-2022
growdiaries.com/giveaways/FastBuds-Summer-Buds-2022
growdiaries.com/giveaways/DYNOMYCO_Show_Us_What_You_Got
growdiaries.com/giveaways/BDotY-2022
All these contests are the special ones that do not repeat monthly. Some of them you need to be selected by the host of the contest and they you can join once you get their permission.
These ones are almost always specialized int hat you need to use their products.
Read every contest carefully as the rules will say who, what, where, when, why and how, If they don't, or its not clear to you, ask a question on the contest wall and someone should help yea out. Almost all the contest you have to manually join. the exception to this is GTOM it auto joins you for answering questions.
Good Luck!