General question

I’m fairly new to gaming in general and I’m currently playing Shadow of the Tomb Raider. I just started and I really like it! But I have a question about the gameplay. I’m following the storyline from the beginning but as I got further it was also possible to explore the environment and gather new items. I noticed some tombs and areas but I couldn’t get in because I didn’t have the right gear or I was following the storyline. I’m wondering now, can I go back later to certain areas to explore more and enter the tombs? Or am I ‘supposed’ to explore while I’m there and will it be shut after? I hope you can help me out here! (English isn’t my first language do excuse me if I used the wrong words)