Thanks for your questions.
I think I see badly soldered pins on the micro board, and some pins not soldered at all.
I can confirm all the pins are soldered properly. The devices work just not together.
Don't know these micro boards, but the schematic diagram shows that "V-in" (pin1) is USB supply through a backflow protection p-channel mosfet. This (naming) is different from regular Arduinos.
The HC-06 module is being powered by VIN as it disrupts the rest of the module when connected to 3v3. The modules data sheet says it is rated for supply anywhere between 3.6 and 6 V
But why power the SD from the 3.3volt pin of the micro (with a tiny onboard 3.3volt regulator), when the SD card has it's own regulator. Just power the SD card (5volt pin) also to the V-in pin of the micro.
This is a 3V3 based application. In the end, I will be using a 3.7 V battery to power the mcu and regulate the 3v3 voltage using the on board regulator.
Don't know which MPU6050 you have (post links).
SAM D21 mini breakout SparkFun SAMD21 Mini Breakout - DEV-13664 - SparkFun Electronics
0.96 " OLED Display
MAX30102 Heart Rate Sensor https://www.maximintegrated.com/en/design/reference-design-center/system-board/6300.html
MPU6050 Acceleromter 6_DOF_Sensor-MPU6050__SKU_SEN0142_-DFRobot
Note: When I remove the heart rate algorithm, everything seems to work fine. Could the algorithm be taking up too much processing time?