The topic I'm about discuss in this post seems obvious enough - which is probably why it seems to have eluded discussion for some time - but it has quite a few finer points that in many respects do need to be discussed, so there's no better time than now. As many players (Trainers) in Pokémon Go can attest, they can and do need to earn items in the game in order to succeed. Yet in doing so, they also have to work with and around some limits in doing so, notably their own item bag limits. The discussion revolves around both points and how they can take advantage of them in various circumstances.
Most commonly, every Trainer knows of spinning PokéStops to get items to fill and overfill their item bag. However, what they may not know is that they can still spin PokéStops when their item bag is filled to its exact capacity, and if they haven't claimed a free raid pass - that comes one at a time - they can spin the Gym photo to earn items and the free raid pass, regardless of whether their bag is filled or overfilled. This last bit is incredibly useful as it still counts as a PokéStop spin and can be taken advantage of while a Trainer still has an overfilled item bag, for example after leveling up and getting items that way.
Gifts from other Trainers are a certain source of items, and they'll certainly fill and overfill item bags as well. Opening a Gift to get items when the item bag is filled to its exact capacity is also possible, but opening Gifts after that point will only award Stardust as has been the case since a few years ago, or if consent hasn't been made for that to occur, just a simple "item bag full" warning. This is certainly one case where Trainers really have to work with their item limits to ensure what and how many items they can obtain.
Certainly, there are the game shops as sources of items. Bonus items such as those from the Reward Road and purchase of some ticket/event packages can be earned regardless of item bag content, but actual packages (boxes) of items - especially those with very many items in them - including the Daily Free Box will require sufficient item bag space to obtain them, and a warning to make space for them (if not already implied by a "bag full" warning) will be given otherwise. It's something to consider when planning the purchase.
What remains quite evident is that items are necessary to be obtained in Pokémon Go in any way possible to make things progress. Some limits - as the item bag limit primarily - may affect how Trainers can obtain items in all kinds of situations, but it can be seen that there are still ways to work with and around them to obtain the necessary items. After that, it's just a matter of using the items for the best of successes.
One year ago: Pokémon Mornings and Evenings
Two years ago: Recent Pokémon Go Interface Changes
Three years ago: Cosplay: Lippo Bunkasai Japan Fest
Four years ago: (Possible?) Community Day Concepts
Five years ago: Preparing for Pokémon Go Tour
Six years ago: For the Sake of Consistency
Seven years ago: Pokémon Go Event Rollup, 2/19/2019
Eight years ago: Pokémon Go Team Leader Stickers
Nine years ago: Let's Get Ready to Rumble