Since I'm passionate about this topic, I would like to add my opinion to this thread as well. I would like to preface this post and say that I'm not an expert or a professional in this field. However, I have consulted a lot of (in my opinion) trustworthy sources, such as dermatologists, cosmetic chemists, and other professionals in the beauty industry. So while I cannot vouch for 100% accuracy for what I'm saying, these are recommendations that most, if not all, of my sources, have given.
The general recommendation is to wear a broad-spectrum sunscreen with at least SPF 30 every day. Personally, I prefer to use an SPF of 50, which I try to put on every day. I hope the reasons why I prefer this higher SPF will become apparent when I'm done with this post
I think that wearing an SPF every time you step outside is important. However, even if you're staying inside all day, wearing sunscreen might still be appropriate. On sunny days, the amount of UV radiation (especially UVA) that is transmitted through a window can still be significant, as windows don't reflect this type of radiation reliably. This means that even sitting by a window on a sunny day can cause sunburn if you're not protected.
When you put on your sunscreen, it should, ideally, be the last product you use. While this might not be the case for makeup wearers, sunscreen should still at least be the very last step of your skincare routine. In order to work properly, the UV filters must form an even film over your skin. Every time you apply something on top of this film, you run the risk of disturbing it, thus reducing or even eliminating its protective properties. The film-forming is also the reason why it is important to apply enough sunscreen. If you apply less than the recommended amount, the UV filters might form a thinner film, which will reduce the effective SPF of your sunscreen. If you apply too little, a film might not even form, resulting in very spotty protection which can leave regions of your skin exposed to radiation. To get the SPF on the label, you'll have to apply 2 mg of sunscreen per cm^2. While this might not look like a lot at first glance, it does translate to about a quarter of a teaspoon for your entire face, which is a considerable amount.
This is also the reason why I would recommend staying away from sunscreen powders as your primary protection. To get the desired SPF, you would have to apply a lot of powder, resulting in a powdery, cakey finish. People just don't apply enough powder to get significant sun protection.
Depending on what you apply underneath your sunscreen, the sheer amount of product on your skin might not be very comfortable for you. Therefore, it is important that you find a sunscreen that is right for you. If your skin is oily, you might want to skip a separate moisturizing step if you can find a hydrating sunscreen that feels good to you. If you find that this doesn't quite cut it, you might consider just layering a hydrating serum underneath your sunscreen (UV filters, especially the inorganic ones, tend to be occlusive). For slightly dry skin, your skin might be very happy with just a nice, moisturizing sunscreen. This will be different for everybody, and you'll have to experiment with what feels right for you.
When you expect a longer time of sun exposure, it is also important to reapply your sunscreen in accordance with the instructions on the package. The reason for this is two-fold: Sunscreen will naturally wear off over time. Every time you touch your face, every time your skin comes into contact with your clothes, a small amount of the UV filters on your skin will be removed. Sweating (or becoming oily) can accelerate the wear-off. UV filters are also not photostable over the course of an entire day. So even if there was no physical removal of UV filers, they will degrade as they work to protect your skin.
Which UV filters you choose to use is up to you. Generally speaking, there are two categories of UV filers: inorganic and organic ones. There are only two inorganic UV filters, zinc oxide and titanium dioxide. The rest of the available UV filters are classified as organic. You might have also heard the distinction between a "chemical" and a "physical" UV filter. These terms, while widely used, are very misleading. First of all, as I'm sure you're aware, the distinction of chemical and non-chemical ingredients is non-sensical, since everything (even water) is a chemical. Secondly, the term "physical" refers to the process of how these filters allegedly work. "Physical" filters (these are the inorganic filters) supposedly work by reflecting the incoming radiation and thus preventing it from reaching the skin, while "chemical" filters work by absorbing the radiation and saving it in other degrees of freedom (such as vibrational ones). This is a very long-lived myth, one that even professionals still sometimes believe in. The reality is that while inorganic filters are capable of reflecting UV radiation, this only accounts for about 5-10% of their filtering capabilities. The primary filtering process is absorption, just as is the case with organic sunscreens.
Generally speaking, inorganic filters are recommended for people with sensitive skin. This is because the older organic filters (such as avobenzone) tend to be more irritating than the inorganic filters. However, newer organic filters tend not to have this issue, so even people with sensitive skin might be fine with more modern organic filters.
Sunscreens with inorganic filters can sometimes be unpopular because of the white cast they leave behind. Both zinc oxide and titanium dioxide are white pigments, which is why they may leave a white cast. If this is a concern for you, and you still want to use inorganic filters, a tinted sunscreen is a possibility.
I hope this helps!